Notes on the petrography of the Northampton Ironstone

375

NOTES ON THE PETROGRAPHY OF THE NORTHAMPTON IRONSTONE. By]. G. A. SKERL,

~I. Sc.,

1'.G.S.

[Received 26th Nov ember, 1926 .J [Read ISI April, 1927 ·J

I.

Introduction.

lN of19 his4 Professor P. G. H . Boswell (1)* published the results investigations on the petrography of the Inferior 2

Oolite sands of the south-west of England. The present notes represent part of the results of similar work on the constitution of beds of approximately the same age in the Midland counties, carried out at the suggestion of Professor Boswell. At the commencement of the work it became apparent that the succession of the beds concerned was so variable as to necessitate the adoption of a datum-line. Accordingly the basal portion of the Northampton sands (the Northampton Ironstone) was used for this purpose, a course made obvious by the uniformly argillaceous character of the Lias, together with the lack of its exposures. The Northampton Ironstone occurs over the greater part of the outcrop and was opened up extensively during the war for exploitation as an iron ore (2). Although the Northampton Ironstone has been seen so far as fourteen miles to the north of Lincoln (at Blyborough) it is of no great thickness, whilst very variable in character (fig. 43). But at Greetwell, one mile north-east of Lincoln, the bed attains an average thickness of 9 feet, and has been worked to a large extent for mixing with the more calcar eous Frodingham ore . Th e ironstone rapidly thins t o t he north, east and west of Greetwell. South of Lincoln, in t he a rea ext ending to near Lead enham, the working of the ore would not be a profitable undertaking, for its occurrence is sporadic and the silica-con tent too high. The ore is quarried at Leadenham, but although there appears to be ample reserve under cover south of that place the ironstone is not worked again un til St ainby is reached, where the main ore-field begins. At this place the outcrop becomes wider, mainly because of the small angle of dip and the removal, by erosion, of the overlying Lincolnshire Limestone. The ore is found to become siliceous both towards the west and also eastwards, where it passes beneath the cover of more recent beds. Nevertheless, the composition and charact er of the worked ironstone remain remarkably constant as far as the neighbourhood of Northampton. H ere it becomes thicker (at Duston some 25 feet have been worked, in comparison with the usual 8 feet) but further west• The numbers in bra cket s refer to the bibliography at the end of the paper.

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THE PETROGRAPHY OF l'iORTHAMPTON IRONSTONE.

377

wards and south-westwards, as, for example, at Stowe-nineChurches, it passes into a very arenaceous and calcareous rock. At Steeple Aston in Oxfordshire a shaft was sunk for the purpose of winning both the Inferior Oolite and the Marlstone ores, which were only 80 feet vertically apart, but the working was soon closed down. Except at Greetwell and Irthlingborough the ore is won in open-cast quarries; at these two places the ironstone is mined. The precise stratigraphical position of the Northampton Ironstone has yet to be determined (at least, according to the requirements of modern Jurassic standards). The work of Dr. A. E. Trueman on the Lias of South Lincolnshire (8) shows, however, that the Northampton Sands are non-sequential in relation to the Upper Lias, and that they lie on successively higher zones as they extend from Lincolnshire to Northamptonshire. The Northampton Sands contain as characteristic fossils Lytoceras wrighti, S. Buckman, L. opalinum (Rein.) and Ludwigia murchisonce (J. de C. Sow.), and represent the upper part of the Jurensis zone, as well as the Torulosus, Opalinus and the lower part of the Murchisonee zones. While as yet no evidence has been adduced that the deposit crosses time-planes, as in the case of the Lias-Inferior Oolite sands of the south-western area, it is possible that this is the case. For various reasons the junction of the Northampton Ironstone (forming the basal component of the Northampton Sands) with the Upper Lias is rarely seen. For example, the base of the bed is usually ferruginous sandstone of no commercial value, and therefore not exposed in working, or sometimes the base is too calcareous or argillaceous. When good ironstone lies directly on the Liassic clay, a foot or so is often left to facilitate drainage and working. II.

Megascopic Characters.

The Northampton Ironstone, whilst preserving as a deposit a very constant character over all its outcrop, shows in handspecimens considerable variations. In the unweathered state it usually appears to be a bluish-green oolitic and argillaceous ironstone, which on weathering loses much of its oolitic character and becomes a limonitic ore. A proportion of sand is always present, and calcium carbonate is a persistent though variable constituent of the unweathered rock. A feature of the Northampton Ironstone is its tendency to weather into rectangular blocks or " boxes," usually with a core of unweathered ore and an outer layer of dark brown limonite. which becomes less marked as the centre is approached. When the process of weathering is completed, instead of a green core there remains a pale centre of earthy material surrounded by PROC. GEOL. Assoc., VOL. XXXVIII.. PART 3, 1927.

25

J.

G. A. SKERL,

'Concentric boxes of dark limonite. A diffusion of material appears to have taken place in the manner of Liesegang rings. The average chemical composition of the ore in the unweathered state is :-metallic iron, 30%; silica, 14% ; alumina, 6% ; and lime 3%. These are combined with carbon dioxide and water to form iron carbonate (siderite), calcite and a green hydrated aluminous iron silicate (chamosite); a varying proportion of sand and clay is also present. The weathered product 'Consists of limonite, sand and earthv material and contains a higher proportion of metallic iron. The ironstone is essentially siliceous, hence its mixture with the calcareous Frodingham and marlstone ores to yield a self-fluxing charge. The content of phosphorus is high, but the proportions of sulphur and manganese are low.

III. Petrography of Finer Constituents. Representative samples of the exploited Northampton Ironstone of about SIbs. weight were collected wherever possible, so that a total of some 38 specimens was obtained. In a few cases samples were taken from the top and the bottom of the bed in order to ascertain if there was any vertical change. Samples of unweathered and weathered ore from the same quarry were also examined. Portions of the samples were crushed so as to pass a I mm. sieve; some were not crushed, but dissolved in the condition as collected, in order to see if crushing splintered the grains. This, however, was not the case. The material was dissolved in I : I hydrochloric acid, and in most cases effervescence in the 'Cold showed the presence of carbonate minerals (probably 'Calcite). On warming there was further effervescence, due to the presence of siderite in the sample. The heating was carried out on a water bath or hot plate, and boiling was avoided in order that other minerals should be affected as little as possible. The addition of a small amount of stannous chloride facilitates the solution of these highly ferruginous rocks. In many cases carbonaceous matter rose to the surface during the treatment, thus showing that the ore contained organic matter. In all probability this was of vegetable origin. The absence of any evolution of sulphuretted hydrogen appears to indicate that pyrrhotite was not present. The warm acid bleached the clay substance and oolitic grains. Only the oolitic grains containing some mineral matter remained behind when the ferric chloride and other products of solution. together with clay, were decanted out of the dish. A sandy residue remained, and a good" pay-streak" of heavy minerals was clearly seen, the proportion being undoubtedly high, sometimes amounting to IO per cent. The amount of sand

THE PETROGRAPHY OF NORTHAMPTON IRONSTONE.

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obtained was variable, according to the sample and place of origin. Separation into heavy and light crops by means of bromoform of density greater than 2.8 followed. The heavy crop was then separated magnetically and electro-magnetically. The following list of minerals only includes those found in the sandy residues after the solution of the ironstone. Therefore, it does not include such minerals as calcite, siderite, chamosite, -etc., which are affected by the acid treatment. CUBIC.

Garnet Magnetite Pleonaste Pyrite ORTHORHOMBIC.

Andalusite Brookite Hypersthene Sillimanite Staurolite Topaz

TETRAGONAL.

Anatase Rutile Zircon MONOCLINIC.

Biotite Chlorite Chloritoid Epidote Glaucophane Hornblende Monazite Muscovite Orthoclase Sphene

HEXAGONAL AND TRIGONAL.

Apatite Ilmenite Quartz Tourmaline TRICLINIC.

Kyanite Microcline Oligoclase

CUBIC.

Gar net is by far the most abundant heavy mineral, and occurs in chips of average diameter of 0.2 mm., and as rounded grains up to 0.8 mm. in diameter. The larger grains vary from sub-angular to rounded, and are often of red (pyrope) or purplish (almandine) tinge. The smaller particles are angular and are colourless to pale pink. The angularity and embayments in the grains are often remarkable. Inclusions are not common, but cavities occur frequently in the large pink garnets. A few pink grains contain hosts of rods which appear to be of sillimanite. The small chips of pale garnet are typical of the Inferior Oolite over the whole of its outcrop. The larger and more deeply coloured varieties are most abundant in the Kettering area, few being found in the northern portion of the outcrop. Mag net i t e is more abundant than ilmenite, and sometimes the octahedral shape can be seen, even though the grains be slightly rounded.

J.

G. A. SKERL,

PIe 0 n a s t e is a persistent mineral, one or more grains occurring in nearly all mounted residues. It is usually in the form of brilliant blue-green chips about O.I mm. in diameter, although some grains show evidences of the octahedral form. P y r it e forms the bulk of the heavy residue from the unweathered ironstone, so much so that it has to be removed by means of nitric acid in order that the detrital minerals may be examined. It is absent from the weathered ore. Usually in the form of shapeless aggregates it does, however, appear as nodules and as cubes, and always shows its characteristic brassy yellow colour in reflected light. That the mineral might be marcasite has not been forgotten, but the fact that it has been found in samples of green rock, which have been exposed to atmospheric conditions for a period of sometimes as much as eight years, seems to indicate rather that the sulphide belongs to the more stable form, pyrite. Whether any of the pyrite was detrital could not be ascertained, but this is considered improbable. TETRAGONAL.

A nat a s e is a rare mineral, only three grains having been seen. These were yellow plates, O.I mm. in length with the high refractive index and uniaxial negative directions-image of the mineral. Rut i I e is a very abundant mineral and occurs in both magnetic and non-magnetic crops. Although usually in the form of yellow, foxy-red and deep-brown needles and prisms of (probably) authigenic origin, both the foxy-red and yellow varieties have been found in the ooliths, showing that they may be allothigenous. The red and brown varieties predominate. Sagenite webs of yellow colour are met with, some still surrounded by mica, but one flake of the latter mineral enclosed a fan-shaped aggregate of red needles. Twin crystals of various types occur, whilst some examples of rutile growing on ilmenite are to be seen. Zi reo n. This mineral is, after garnet, the most abundant heavy species, and is found in all possible forms and colours. It is interesting, however, to note that whilst the variety is not abundant, rose-purple zircons occur and are usually well rounded, although one prismatic grain 0.2 mm. long was noticed. RHOMBOHEDRAL.

A pat i t e. Several grains have been found in highly calcareous ore from Greetwell. Occurring in the form

THE PETROGRAPHY OF NORTHAMPTON IRONSTONE.

38I

of stumpy prisms, with fairly high refractive index and low birefringence, the mineral probably owes its preservation to the easy solubility in acid of the rock matrix. I I men i t e is not so abundant as magnetite in the northern portion of the outcrop, but increases in quantity when the area around Finedon is reached. The lustre as well as its weaker magnetic properties serve to distinguish it from magnetite. Qua r t z is, as usual, the most abundant mineral, although sometimes felspar challenges its predominance. The angularity of the grains is often remarkable, and shows that the material could not have travelled far, that it must have had its irigin near the outcrop of the Northampton Ironstone and that the mineral assemblage is probably that of a first cycle sedimentation. The grains occur up to 2.5 mm. in diameter, the larger grains being usually well rounded. Inclusions of zircons and carbonaceous matter are seen. To u r m a lin e is not as plentiful even as staurolite and sphene, and, contrary to the rule as regards most British sediments, is a subordinate mineral. It occurs chiefly in irregular grains, usually rounded to a certain extent. Prismatic grains are generally colourless, greenish and purplish, with many inclusions in the interior, probably of magnetite or graphite. The irregular grains are mainly brownish-green, and have, perhaps, survived a second cycle of sedimentation. The fact that the colourless grains are doubly terminated suggests a possible authigenic origin. ORTHORHOMBIC.

And a 1u sit e has only been found at Greetwell in the form of three grains, 0.6 mm. in diameter, and therefore larger than the other constituents of the residue. They show the characteristic pleochroism. The mineral is glass-clear and free from inclusions. B roo kit e. Several grains of this easily-recognised mineral have been found. The occurrence is sporadic, but in each case the grains occur in residues from which sphene is absent or has undergone much decomposition. The distinctive biaxial positive directions-image was obtained, showing the strong dispersion. H y per s the n e. A single grain of this mineral was seen from Cottesmore. Sill i man i t e was only seen definitely as inclusions in some of the pink-red garnets.

J.

G. A. SKERL,

S tau r 0 lit e occurs in all the residues. Usually it is in the form of yellow-brown to red-brown grains, but occasionally unworn flakes of more delicate golden yellow were found. In some residues from the southern outcrop there are also large rounded grains up to 0.7 mm. in diameter, apparently of different origin. Raggedness is extremely common and is due to decay along cleavage planes. The darker variety seems to have withstood this decomposition better than the lighter coloured type. Top a z occurs sporadically in irregular chips up to 0.3 mm. in size. Rare basal cleavage flakes have been seen, but otherwise identification is always a matter of some difficulty. The chief characters shown are the fairly high refractive index and low order interference colours. MONOCLINIC.

Bi

tit e. A single flake of a brown mineral bounded on three sides by straight lines and on the other by an irregular fracture, green-brown in colour and slightly pleochroic, favours identification as biotite. The pseudo-uniaxial negative directions-image yielded seems to confirm the identification. ChI 0 r i t e occurs sporadically in many residues from the central area of the outcrop. The presence of more than one variety is indicated by the optical properties. Penninite, with its ultra-blue interference colour and pseudo-uniaxial positive directions-image, was encountered, but delessite was the commoner variety. ChI 0 r ito i d. A mineral which occurs in irregular cleavage flakes in all residues with a "muddy" pleochroism from sea-green to sea-blue was suggestive of chloritoid. Further confirmation was obtained by the refractive index, which was slightly below that of a mixture of methylene iodide and O'-monobromnapthalene (!-t = I.7 I S), the low birefringence and the presence of a central biaxial positive directions-image showing a fairly well-marked dispersion. A second cleavage was poorly developed in some flakes. A few grains bounded by this 010 cleavage showed pleochroism from pale green to sea-green. Hence X (b) = sea-green, Y (a) = sea-blue, and Z (c) = pale green. Inclusions are not common. The mineral is present in all residues, but in thickness of flakes and abundance seems to show a progressive decrease southwards. 0

THE PETROGRAPHY OF NORTHAMPTON IRONSTONE.

383

E P i dot e is a comparatively rare mineral and is found only in the form of small yellow-green rounded grains of high relief and birefringence. It simulates monazite, but is distinguished from that mineral by the absence of a characteristic spectrum. G 1a u cop han e. This very distinctive mineral occurs sporadically as cleavage flakes. The pleochroism is X (c) = pale green, Y (b) = purple, and Z (a) = royal blue. The extinction angle is about 5°. Angular grains and cleavage flakes occur at Stainby, but southwards the mineral becomes more irregular in shape and shows the raggedness due to the rapid disintegration which such a highly-cleaved mineral must undergo. H 0 r n b 1end e. This mineral is very rare. It is usually in the form of elongated and wispy grains possessing pale to darker green pleochroism. The extinction of ISO points to the presence of actinolite. M 0 n a zit e was only definitely proved by spectroscopic examination in the case of one small rounded grain, which also gave a positive directions-image with a low optic axial angle. Mus c 0 v i t e is not an abundant mineral and occurs sporadically. The flakes are thin and show strainpolarisation effects. Another micaceous mineral having one refractive index just above Canada balsam (1.54) and the other indices about those of muscovite, is common in slides of residues from the Midland portion of the outcrop. The birefringence is stronger than that of muscovite, and though the directions-image is similar (centric biaxial negative), the optic axial angle is less, varying from about 5° to 20°. Moreover, inclusions are found within the mineral along certain definite parallel planes perpendicular to the basal cleavage, afeature uncommon in muscovite. The mineral is similar in some respects to that described by Professor P. G. H. Boswell from the Inferior Oolite sands of the south-west (1) and may be one of the brittle micas. o r tho cia s e forms the greatest part of the Ielspars present. Although usually in the form of sub-angular grains, in some light crops it exists mainly as cleavage flakes. S p hen e occurs in all the heavy residues (except that from the northern portion of the outcrop) in both magnetic and non-magnetic crops, but chiefly in the latter. The non-magnetic crop at Stowe-nine-Churches

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consists of little else but sphene and kyan it e. It is found as colour less, pale greenish-yellow or brownish eq uidimensiona l grains up to 0.3 mm . diam et er , showing, in some cases, abra ded angles. The mineral has a high refracti ve index a nd variable birefrin gence. Th e absen ce of ext inct ion du e to its high dispersion renders the min eral easy of det ecti on . E vid ence of a good biaxi al positi ve directi ons-im age is usually obtained. As the out crop is t raced north- and sout h-westwards from the Irthlingb orou gh-Northampton ar ea the min eral is seen t o suffer increasing decomp osition, it s final appearance being in the form of irreg ula rly et che d cleavage flakes and gra ins covered with alte ra tion products. "T R ICLIN IC.

K 'y ani t e. This mineral occurs in all residues in grains up to I mm. in size, but is rare in the sam ples from Lincolnshire. It is of ext reme interest, as it shows all transitions fr om the un alt ered material t o t he merest threads. Th e genera l mode of occurrence is that frequently met with in t he Lower Greensand and other deposit s-stumpy pri sms showing the clea vages very pla inly. On wea thering , however, the prominent pinacoidal clea vage causes the formation of lengthy, almost wisp-like, forms. As is t o be expecte d , kyanite is first attacked along the clea va ge cracks . At this stage the mineral may have the bluish ti nge t yp ical of most hand-specimens, and it would seem t hat this colour at ion is du e to incipien t alteration alon g the cleavage planes. Th e min eral next appears to split and th in flak es become numerous . Usually they have serra te d edges due to decomposition. The chemi cal decay continues and comparat ively large hol es are eate n int o th e sides of th e grains, which are sometimes covered with decompositi on product s. The latter giv e the appearance of a higher birefringence. As this alteration progresses the mineral breaks up more rapidly alon g it s prominent clea vag e, forming the long wispy skelet ons referred to above. The " motheaten" forms do not seem to have been recorded previously. Kyanite occurs in it s most abundant and unaltered form in the Irthlingbo rough-Northampton ar ea , north and south of which it det eriorates in size, quantity and quality.

T H E PETROGR APH Y O F ~ORT HAM PTON IR O NST ON E .

385

111 i c roc lin e is seen frequently as an gular fragments. The characte ristic cross-ha tc hing is rar ely seen, but grains showing multiple twinning and havin g a refracti ve index just below 1.52 mu st be referred t o microclin e, the mineral lyin g on th e (010) cleavage. Th e extinct ion is about 5°. o 1i g a c i a s e is the only plagioclase felsp ar present . Twinning is never shown and the identifica tion is only rendered possible by t he use of liquids of kn own refra cti ve ind ex . Th e grai ns have a refract ive index above 1.53, but ju st below 1.548. Doubtful iden tificati ons includ e those of cassit erite , pyroxene and zoisite. With regard to the first-nam ed, it is worthy of record that , on crushing a specimen of granite from Mount Sorr el and separating the he avy minerals by mean s of br omoform, the greater portien of the non-magneti c crop is seen to be composed of cassiterite with a distin ctive blood-red to br own pat chy pleochroism. Notes on the Heavy Minerals. Th e assemblage of hea vy minerals is typi cally that of most of the Upper Liassic-Inferior Oolite deposit s of the Midland a nd Southern ar eas of depositi on . The present investi gation ha s r esulted in the first syste ma tic recognition of chloritoid (1). I t is true that Prof. A. Holmes, in his book P etrographic M ethcds an d Calculations , p. 191 , records the mineral in a table as occurring occasio na lly with t opaz in th e Inferior Oolit e sands of Northampt onshire, in which ilmenite, garnet, and st aurolite ar e given as abundant ; magnetit e, t ourmaline, glauconite, rutile, zircon and mu scovit e are common; while anatase and kyanite are rar e. Th e results of my own investigations on t hese sands disagree with this analysis, for t he mineralogical consti t ut ion changes with the different beds of thi s variable series of deposit s. Chlorito id is always present in the Northampton Ironstone h eavy residues. Th e angularity of mu ch of th e quartz and felspar, and th e abundance of the latter , t estifies to the proximity of the source of origin and the presence of only one cycle of sedimentati on. The fr eshn ess of many of the min erals also tends to support this view. The rarity or almost complete absence of the isom ers, brookite and an at ase is of considerabl e interest, for these min erals were found in the Cleveland Ironst one by Mr. C. R. Lindsey (3), but in the magnetite ore at Rosedale, of nearly the same age as the Northampton Ironst one, brookite was ab sent. In the Northampton Ironst one it would seem t hat th ese minerals are in inverse

J.

G. A. SK ERL,

proportion to sphene , th e degradation of whi ch would set fr ee titanium dioxide as well as calcium salts and silica. The abundance of sphe ne is not eworthy and remarkable, since this mineral, although common in plutonic rocks, occurs but s poradically in sediments . The euhedral crystals found by Dr. R H. Rastall in the Lower Greensand at Aspley Guise (4) should be noted as throwing some light on the origin of this min eral, but the habit of the min eral in the ironst one is gra nular and similar to that found by Professor P . G. H . Boswell in the Li asInferior Oolit e sa nds of the West Count ry (1). The shape of the grains of tourmal ine, zircon or staurolite , apart from their relative abundan ce and ot he r pr operties, does not ad mit of the possibility of their being deri ved from Triassic deposit s. Moreover, the Tri as was probably totall y sub merged during the Liassic transgression , and would not have been ex posed. The heavy minerals of the ironstone seem rather t o be the pr oducts of a first cycle of sed iment ati on . The heavy mineral assemblage of the Northampton Ironstone a t Lin coln consist s (in order of abundance) of pale garnet , zircon, staurolite, chlorit oid, kyanite, spinel and tourmaline, but south of Lin colnshire it is enriched first by further k yanite and then by sphene and ri cher coloure d garnet s. In mid-Northampt onshire the asse mblage is re presente d , in orde r of abundance, by garnet, zircon , kyanite, sphene , tourmaline, chloritoid and spinel, whilst farther t o the south-west kyanite and sphene show sign of degradation. This degradation of heavy minerals pr ogressing away from their source of orig in, especially of kyanite , has been noticed by Dr. H. H. Thomas (5) and Professor P. G. H . B oswell (1), and is of grea t importance in determining their provenan ce. In the case of the Northampton Ironstone the ver y rapid degradation of both sphene and kyanite away from a centre, lends st rong su pport to the view, if any were needed , that the ironsto ne had it s origin in balanced che mical reactions, causin g precipitation of chamosit e, siderite, calcite and pyrites, all, or some of whi ch , constitute the un weathered ore . The reader is referred to the ad mirable me moir by Mr. A. F. Hallimond (6) for adeq ua te discus sion of t he origin of such iron-ores as that under considera tion .

IV. Palmogeography. The existence of the asse mblage of minerals, notably of garnet, kyanite, sphene, chloritoid, spinel and st aurolite (all minerals of thermal metamorphism) in the Northampton Ironsto ne, impels speculations as t o their provenance, and therefore of the palreogeography of the per iod during whi ch the ro ck was deposited.

THE PETROGRAPHY OF NORTHAMPTON IRONSTO:KE.

387

In considering first the physiographical conditions under which the bed was laid down, we should remember that it has been noted from stratigraphical considerations that, during the Liassic period, an axis of uplift was intermittently asserting itself in South Yorkshire in an approximately east-west direction through the area comprising Market Weighton (7), but this elevation was always less than the rate of deposition. The Liassic deposits thin northwards and southwards from this area. Whether along the line of the Market Weighton axis there is, or was, a full development of Liassic zones it is impossible to say, owing to the lack of precise data on the Jurassic rocks of the district; but if the axis emerged above sea-level, we should have an adequate cause for the palreontological difference, and incidentally the petrographical distinctions also, between the stratigraphical successions of north-east Yorkshire and the more southern development. This is, indeed, most probable, as the work of Dr. A. E. Trueman (8) tends to show that the highest zone exposed along the Market Weighton axis is one belonging to the Whitbian, since the basement-beds of the Inferior Oolite lie non-sequentially on lower and lower zones as the outcrop is followed northwards. For example, the highest Liassic zone exposed in Northamptonshire is lilli, at Grantham fibulatum, and at Lincoln sub-carinaium, But on the northern side of the Market Weighton axis on the Yorkshire coast there is a complete succession of Liassic deposits up to at least striatulum, and the Blea Wyke Beds are of probable Dumortiera date. Thus it would seem feasible to believe that the Market Weighton axis was covered by Liassic deposits reaching to striatulum. Therefore, the rise of the axis would sever part of the Lias and Inferior Oolite from a northern and north-eastern supply of detrital minerals; moreover, the region of the axis itself would probably be eroded, either by sub-serial or sub-marine agencies, to form other deposits southwards. The study of borings, etc., points to the significant conclusion that the greatest development of Liassic deposits in this country is to be found beneath the area covered by the Northampton Ironstone, that is in a belt passing approximately through Northampton, Kettering, Stamford and Lincoln. To the east and south-east of this area the Lias thins out until it borders on to the Palseozoic land-mass. To the west and north-west it also decreases in thickness, but erosion and other agencies deprive us of a definite shore-line; the evidence, however, of the isolated outliers in Shropshire and Cumberland shows that it was probably far distant. Into this area of great deposition, detritus poured from the Carboniferous, Devonian and older rocks, occurring under the east and south-east of England.

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G. A. SKERL,

Owing to the usual lag of isostatic adjustment there was still a continued depression towards the end of the [urense hemera in the Midland area when there was a general uplift ushering in the Inferior Oolite period. In this depression the Northampton Ironstone was formed, for if the boundary of the deposit be sought one finds that it consistently becomes more arenaceous and rapidly disappears into a ferruginous sandstone, and finally into a few inches of sand, which always rests on Lias clays. Hence the ironstone was deposited in an epicontinental basin circumscribed everywhere by argillaceous Liassic beds. The boundaries of this basin are roughly shown in fig. 43, except in the northern portion, where denudation has removed the greater part of the deposit. From all sides of the basin detritus poured in, and since most of the exposed strata were argillaceous, the streams could have carried little else but clay and products resulting from the atmospheric disintegration which the exposed beds must have undergone. Judd (9) describes upright plant remains in the ironstone from Rutland, whilst solution of the rock reveals carbonaceous matter. Conditions were not favourable to animal life, which is represented only by shell-beds at the bottom and top of the limestone, evidently deposited in cleaner water. Whence were the detrital minerals derived? The grainsize of the detrital minerals is consistently greater than that of the underlying and surrounding Liassic strata; they must therefore have had their provenance in a coarser sediment, or directly in an igneous or metamorphic area. The angularity of the grains, coupled with the freshness of the minerals, shows that the assemblage is one belonging to a first cycle of sedimentation. The study of the frequency of occurrence of sphene and kyanite shows that these two minerals increase southwards both in size and quantity until the Irthlingborough and Northampton regions respectively are reached; they then become rarer and more disintegrated as the outcrop pursues its way west and south-westwards. An east or south-easterly origin is thus adumbrated for these minerals. Evidence is afforded at Irthlingborough that a stream of some consequence entered the basin, since the ironstone there is mined because of its being mainly sideritic and not chamositic, due probably to the fact that the stream contained a high percentage of soluble carbonates. The occurrence of " wash-outs" and similar phenomena, as well as the shape of the outcrop, in the district just south of Stamford, offers evidence of the outpouring of a large stream of detritus into the area of deposition at this point. Considering in some detail the provenance of some of the more determinative heavy minerals, we may regard chloritoid

THE PETROGRAPHY OF NORTHAMPTON IRONSTONE.

389

(because of its comparative rarity) as possibly the most valuable for our purpose. Chloritoid is perhaps best known from the metamorphosed Salmien rocks of the Ardennes, and it is to that region that one naturally looks for a possible source of origin. According to Anten (10), Dannenberg (11) and Lohest (12) chloritoid occurs in association with garnet, sphene, andalusite, biotite, apatite, zircon, muscovite, epidote, pyrite, pyrophyllite, chlorite, tourmaline, limonite, sagenite, hsematite, pseudocoticule, ottrelite, rutile, ilmenite and magnetite. The extensive collections in the Geological Department of the University of Liverpool provide ample material for the study of residues from the Cainozoic, Upper Cretaceous and Wealden deposits of Belgium. Investigation of these samples, however, showed the absence of chloritoid in areas adjacent to the Ardennes. This result confirms the work of Anten, who also does not find this mineral in the Cretaceous and later deposits. According to the work of Professor P. G. H. Boswell (13) and Mr. 1. S. Double (14) it also appears to be absent from the Eocene of the London Basin and the Pliocene deposits of the East of England respectively. Moreover, J. W. Retgers (15), in his classical and detailed paper on Dutch dune-sands, does not include this mineral amongst the thirty-five he identified. The greater part of the heavy residue of these sands was contributed by the Rhine and Meuse, both of which drain areas which contain chloritoid-bearing rocks. Is it to be assumed that olivine, which is recorded by Retgers, is a more stable mineral than chloritoid, or that the latter has disappeared by deflation in the same manner as mica? A further argument against the Ardennes being the region from which the Northampton Ironstone has been derived is given by our present knowledge of the physiography of the period. At that time the eastern part of Belgium formed the coastal region of a European sea, whilst the area between the Ardennes and the Midlands of England was composed of a Palseozoic landmass, as shown by borings. A recent paper by Dr. C. E. Tilley (16) describes in greater detail the chloritoid of the Cornish peninsula, first noted by W. M. Hutchings (17). The size of the described crystals, as well as different optical properties, considered in relation to the physiographical conditions of the Upper Lias, does not lead to the conclusion that chloritoid was derived from the south-west country. There are two other areas, Scotland and Brittany, in which chloritoid-bearing rocks are known. The Scottish occurrence was first discovered and described by Mr. G. Barrow in 1898 (18). Through the kindness of Dr. H. H. Thomas, of H.M. Geological Survey, who provided me with some of the mineral obtained from the crushed rock, I have been able to examine the Scottish

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G. A. SKERL,

chloritoid and find it to have properties identical with that in the Northampton Ironstone. To Mr. Barrow I am indebted for a sample of the actual chloritoid-bearing rock, in which, however, I was not able to find any mineral similar to the chloritoid under discussion. Professor C. Barrois described many years ago the wellknown occurrence of chloritoid in the schists of Morbihan (19). A specimen of this rock containing large crystals of the mineral was presented by Professor Barrois to the University of Liverpool. The chloritoid is exactly similar to that found in the Inferior Oolite and to the Scottish material supplied by Dr. Thomas. The problem of the provenance of this mineral is therefore one of difficulty and is best approached by a process of elimination. In accordance with the views of Judd (9) it has long been thought that the Middle Jurassic deposits had an origin to the north-west in the ancient rocks of the Highlands of Scotland. Unfortunately the extent of the Northampton Ironstone in that direction has been reduced by denudation, but residues from the present outcrop yield no evidence that there is an increase in quantity or size of detrital material towards the north-west. Indeed, the reverse seems to be the case, for it is from the southeast that enrichment comes, and chloritoid, garnet and staurolite reach their maximum development in the area between Northampton and Kettering, adumbrating an eastern or south-eastern provenance. There are several difficulties in accepting Brittany as the source of the heavy mineral assemblage, the chief of these being the fact that the heavy mineral assemblage in the Inferior Oolite sands of the south-west of England (Upper Bridport Sands) of similar age to the Northampton Ironstone is smaller in grain-size, despite the fact that the sands lie nearer the source of origin. Moreover, the beds of similar age in Somerset, Wiltshire and Gloucestershire are mainly marine limestones, whilst the Northampton Ironstone is a brackish-water deposit. Kyanite, as shown in Professor Boswell's list of minerals, increases towards the Midland area in the sands of Somerset and Gloucester, whilst in the Northampton Ironstone the percentage of kyanite is at its maximum near Irthlingborough, decreasing both northwards and to the south-west, thus showing a connexion between the two deposits. The evidence thus adduced from the study of one particular mineral, chloritoid, seems to indicate an origin to the east and south-east of the present area of the Northampton Ironstone outcrop. As this area must, from the character of the detrital minerals, be of considerable importance from the view-points of metamorphism and igneous activity, it is interesting to note that Dr. R. H. Rastall in his work on the Lower Greensand (4)

THE PETROGRAPHY OF NORTHAMPTON IRONSTONE.

391

only found garnet in Norfolk and south-westwards to Ely, whilst sphene was only found at Aspley Guise, some twenty miles south of Finedon, where this mineral reaches its maximum in the Northampton Ironstone. In this connection it is also noteworthy that at the base of the Calvert boring (20) Tremadoc shales were encountered, cut by dykes and sills of altered olivinedolerite of considerable thickness. This would seem to indicate the presence of a province of igneous action and perhaps metamorphism of post-Cambrian age, possibly much later, especially if the presence of galena in the Old Red Sandstone proved in the Richmond boring be connected with the same zone of activity. Thus there would seem to have been exposed at the end of the Liassic period a zone of metamorphism in or to the northeast of what is now termed the London Basin, and also an area of igneous or metamorphic rocks towards the west of that region. The following supplementary facts may also point to a possible area of metamorphic rocks lying to the north-east of the London Basin. At the bottom of the Stutton boring, near Harwich, some 'slaty rocks were encountered which recalled to M. Mourlon the phyllades of Vellers, near Ath, of Lower Salmien age (21). An examination of an Oolitic sand from near Marquise in the Pas de Calais showed the presence of both chloritoid and kyanite with garnets and staurolite and a preponderating percentage of zircon. The grade-size was small, as was also the percentage of heavy residue. The Oolitic strata of this area are transgressive on to the Devonian and Carboniferous, and no doubt much of the zircon was derived from these rocks. The relative proximity to the Ardennes should have given, in comparison with the Northampton Ironstone, only a small relative percentage of zircon, whereas the reverse is the case. The kyanite could only have been derived from Belgium if kyanite-bearing rocks are now covered by later (Cretaceous and Tertiary) sediments. Of this we have at present no proof, and in the absence of such evidence it is reasonable to suppose that the deposit had its origin to the north. Some of the borings west of Dover made during the exploration of the Dover Coalfield (23) have proved the most complete section of Jurassic strata in the British Isles. Here and there certain horizons are unrepresented, but, generally speaking, the area was one of subsidence throughout practically the whole of Jurassic time, the axis of maximum deposition running along the south coast of England. This would have as its continuation a depression of the area around what is now called the Straits of Dover, and to its gradual destruction as a land bridge. But from Domerian to Bathonian times an elevation apparently set in and shallow-water deposits were formed, the sediments being

J.

G. A. SKERL,

derived from an area containing both chloritoid and kyanite, The drainage would have passed over rocks which, as far as heavy minerals were concerned, contained little else but zircon. The abundance of this latter mineral suggests a fairly distant source. From all appearances the residue of the Northampton Ironstone indicates that it belongs to a first cycle of sedimentation. There are but very rare rounded grains of tourmaline, and zircon is of much less common occurrence than garnet, a result on the one hand of derivation from metamorphic rocks, and on the other, of close proximity to the source of origin. Whether the presence of purple zircon (22) is of decisive value in determining the cycle of erosion one cannot yet say, but in the experience of Professor P. G. H. Boswell it is found in residues of most British cycles. The quartz is very angular in many instances, and this character is quite in keeping with the angularity of the flakes of orthoclase bounded by cleavage planes. The proportion of quartz to felspar is, in some light crops, about equal, a fact not to be expected if the felspar had a distant origin or was derived from such a deposit as the Old Red Sandstone (proved in many borings as the Palseozoic floor in the London Basin) and the Coal Measures (proved in the Kent coalfield) which are known to have existed at the surface between the Ardennes and the Midland counties at the time of the deposition of the Northampton Ironstone.

V. Summary. The Northampton Ironstone was deposited in a lagoonlike gulf of the Upper Liassic sea. 2. The most important heavy minerals are garnet, staurolite, chloritoid, sphene, kyanite and spinel. 3. The angularity and freshness of the detrital minerals. denote that the minerals are the product of a first cycle of sedimentation, and that they had a provenance not far distant. 4. Garnet, staurolite and chloritoid are the preponderating heavy minerals in the northern portion of the ore-field. South of Lincolnshire the advent of kyanite, sphene and spinel results in the attainment of an acme, both in number and variety, in the Kettering area. South-westwards the residues show signs of degradation. 5. The curious shape of mineral grains, especially of garnet; sphene and kyanite, even when enclosed in ooliths, points to a chemical mode of origin for the ironstone. 6. Two sources of supply are possible-one from a metamorphic region and the other from an area of igneous rocks. I.

THE PETROGRAPHY OF NORTHAMPTON IRONSTONE.

7.

393

The evidence of various assemblages of heavy minerals as well as that in the Northampton Ironstone shows the metamorphic region to be to the east and the igneous zone to be to the south or south-east of the outcrop.

Grateful acknowledgments should here be made for help received from members of the staff of the Geological Department of the University of Liverpool, especially to Professor P. G. H. Boswell. Mr. C. P. Chatwin, Mr. Linsdall Richardson, and Mr. Beeby Thompson have helped by correspondence and advice, the last-named also by his company in the field. Special samples of materials have been provided by the kindness of the MidLincolnshire Iron Company, Ltd., of Lincoln, and the Leadenham Mines Ltd. Finally, it should be stated that part of the cost of this investigation has been defrayed by a grant from the Royal Society to further research in the Inferior Oolite deposits of the Midland counties.

VI. Bibliography. I. 2.

3. 4.

5. 6.

7.

P. G. H. BOSWELL. The Inferior Oolite-Lias Sands of the West Country. Gcol . Mag., (1924), pp. 246-264. G. W. LAMPLUGH, C. B. WEDD and J. PRINGLE. Bedded Ores of the Lias, Oolites and Later Formations in England. Mcm, Gcol . Suru., Spec. Rep. Min. Res., xii , (1920), pp. 141-207. C. R. LINDSEY. Notes on the Occurrence of Brookite in the Cleveland Ironstone. Mineral. JYlag.. xiv. (1905), pp. 96-98. R. H. RASTALL. The Mineral Composition of the Lower Greensand Strata of Eastern England. Geol, Mag., (1919), pp. 2II-226, 26 5-2 72 . H. H. THOMAS. The Mineralogical Constitution of the Finer Material of the Bunter Pebble-Bed in the West of England. Quart. Journ. Geol. Soc., lviii. (1902), pp. 620-632. A. F. HALLIMOND. Bedded Ores of England and Wales. Petrography and Chemistry. Mem, Gcol, Suru., Spec. Rep. Min. Res" xxix. (1925). H. B. WOODWARD. Jurassic Rocks of Britain, vol. iv. The Lower Oolitic Rocks of England (Yorkshire excepted). Mcm,

Geol. Suru., (1894). A. E. TRUEMAN. The Lias of South Lincolnshire. Gcol, Mag" (1918). pp. 64-73, 101-III. 9. J. W. JUDD. Geology of Rutland. Mem. Ceol. Suru. (1875). 10. J. ANTEN. Contribution a I'etude du Salmien metamorphique du Sud du Massif de Stavelot, dans la region de Recht. Ann. Soc. geol, Belg., xxxv. (1912), Mern., pp. 397-417. II. A. DANNENBERG. Les granites des Environs dAix-la-Chapelle. Ann. Soc. geol. Belg., xxxv. (1909), Bull.• pp. 415-434. 12. M. LOHEST. Sur Ie metamorphisme de la zone de Salm-Chateau. Ann. Soc. geol. Belg., xxxviii. (19II), Mem., pp. II-25, 13. P. G. H. BOSWELL. The Stratigraphy and Petrology of the Lower Eocene Deposits of the North-eastern part of the London Basin. Quart. Journ. Geol, Soc., lxxi. (1916), pp. 536-588. 14. I. S. DOUBLE. The Petrography of the Later Tertiary Deposits of the East of England. Proc, Geol, Assoc., xxxv. (1924), pp. 33 2-35 8. PROC. GEOL. Assoc. VOL. XXXVIII., PART 3, 1927, 26 8.

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23·

THE PETROGRAPHY OF NORTHAMPTON IRONSTONE.

J. W.

RETGERS. Neber die mineralogische und chemische Zusammensetzung der Diinensande Hol1ande. N.J.f.Min., I. (1895), pp. 16-74· C. E. TILLEY. Petrographical Notes on Some Chloritoid Rocks. Ceo!. Mag., (1925), pp. 309-319. \"1. M. HUTCHINGS. On the Occurrence of Ottrelite in the Phyllites of North Cornwall. Geol, Mag. (1889), pp. 214-220. G. BARROW. On the Occurrence of Chloritoid in Kincardineshirc. Quart. Journ. Geol, Soc., !iv. (1898), pp. 149-156. C. BARROIS. Sur les schistes metamorphiques de I'lle de Groix. Ann. Soc. Ceol. du Nord, xi. (1884), p. 18. A. M. DAVIES and J. PRINGLE. On two deep Borings at Calvert Station and on the Palaeozoic Floor North of the Thames. Quart. f ourn, Geol. Soc"lxix. (1913), pp. 308-342. P. G. H. BOSWELL. The Application of Petrological and Quantitative Methods to Stratigraphy. Geoi. Mag., (1916), pp. lOS-Ill, 16 3-169 . \"1. MACKIE.

The Source of the Purple Zircons in the Sedimentary Rocks of Scotland. Trans. Edinburgh Geol, Soc., xl. (1923), pp. 200- 2 1 3. G. \"1. LAMPLUGH, F. L. KITCHIN, and J. PJ{INGLE. The Concealed Mesozoic Rocks in Kent, Mem. Gcol, Suru., (1923).