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jy#—Observations on the Purbeck and Portland Beds,

By THOMAS WEBSTER, ESQ., SECRETARY TO THE GEOLOGICAL SOCIETY.

[Read November 19th, 1824.]

THE following communication is the result of a short visit which I paid to the Isles of Purbeck and Portland some years ago, subsequently to the publication of my Letters to Sir H. Englefield. In that work I traced out the great features of this very interesting district; but a detailed account of the beds is yet a desideratum. To supply in the mean time some materials for such a purpose is the object of the present paper. I shall confine myself here to the beds which furnish the well-known , which is extensively employed in for the side pave­ ment of the streets; and to the beds in the from which the principal material for our public buildings has been procured. The beds in the northern part of the are considerably in­ clined ; they dip to the north, coming out to the day from under a series of clays and sandstones which are the equivalents of the Hastings beds described in a former paper. The situation of all these, and their relations to the chalk, are well seen in Swanwich and Durlstone Bays, in the east end of the Isle of Purbeck, and in Warbarrow Bay on the west. Their general direction being from east to west, a very instructive section of the usually known by the name of the Purbeck Beds may be advantageously studied in the latter of these places. A line of elevated ground, consisting of these calcareous strata, extends from the town of Swanwich to Warbarrow Bay; and the northern part, as far as the village of Worth, is covered with quarries which have long been worked for the London and other markets, as well as for the supply of the neighbour­ hood. Quarries have lately been opened also in the most western parts. In Durlstone Bay, the number and disposition of the beds may be well seen ; on the northern part of the bay they are very regular, but about the middle of the bay they are bent and distorted in a very singular manner* :

* Several drawings which I made from these contortions have been already engraved, and pub­ lished with the letters above mentioned. Downloaded from http://trn.lyellcollection.org/ at University of Glasgow on June 26, 2015

38 Mr. WEBSTER on the Purbeck and Portland Beds. at Tillywhim Quarry, in the south-east corner of the Isle of Purbeck, they may be observed reposing on an oolitic which t consider as identical with part of the series of beds in the Isle of Portland. Here however, and all along the south side of the Isle of Purbeck, from Durlstone Head to St. Adhelm's Head, the strata are horizontal; and many quarries are opened in them for the stone, which is called the Purbeck Portland, and which is in general harder than that in the Isle of Portland. On the remainder of the south coast from St. Adhelm's Head to , near Warbarrow Bay, still lower beds are seen, consisting of clay and dark- coloured compact limestone, which have obtained the name of beds from a village in that district. The Isle of Portland, as is well known, is situated near to Weymouth, and is connected with the land by a bank of shingles, or water-worn pebbles, about 10 miles in length, called the Chesil Bank. Its beds dip also to the north; and on that side, under the great oolitic mass (of which the upper part of the island consists), the Kimmeridge beds are exposed. Having thus exhibited a general view of the situation of the strata which I propose to give some account of, I shall begin by stating some particulars re* specting the Purbeck limestone series. In Durlstone Bay I obtained from the quarrymen the thickness of the se­ veral beds, and the names by which they distinguished them from each other: but I should observe that these local appellations are not used by London ar­ chitects and builders, the whole together passing here under the name of Pur«- beck stone only. The following is a list of the strata of limestone, which are mostly separated from each other by a thin bed of a shivery clay or shale.

Thickness. Thickness. Nos. Ft In. Nos. Ft. In. 1. Marble, not worked at present, uncertain 8. Pitching, stone bed, good " 2. Marble rag 8 0 stone ...... 0 Shiver, marl, &c...... 20 O 9. Tomb-stone bed, good stone 0 3. Single leaper, good stone ... 1 3 10. Yellow bed, bad, not worked 0 0 Shiver, marl, &c...... 2 0 11. White roach, not very good 1 4. Step bed, good stone .... 0 7 White earth, a marie 0 5. Grey rag, good stone .... 0 7 12. Leaper, excellent 1 Shiver, marl, &c. 10 0 13. White bed, excellent 1 6. Toad's eye, a very hard but"* 14. Soft bed, excellent 0 good stone •> 15. Hard bed, excellent 0 Shiver 0 1 16. Mock hard bed . 0 7. Good bed, bad stone .... 1 0 Earth .... 1 Shiver, &c 10 17. Pitcher bed . . 0 Downloaded from http://trn.lyellcollection.org/ at University of Glasgow on June 26, 2015

Mr. WEBSTER on the Purbeck and Portland Beds. 39

Thickness. Thickness. Nos. Ft. In. Nos, Ft. In. 18. Backing 1 3 35. Downs Vein rag Shiver, &c 3 0 Shiver 0 3 19. Royal .1 0 36. Grey bed, No. 2, not worked . 0 8 Shiver, &c 3 0 37. Hone 0 10 20. Devil's bed 0 6 Hone not worked, marly ... 1 6 Shiver, &c 2 0 Shiver 0 4 21. Iron bed 0 4 38. Grey bed, not worked .... 0 4 Shiver, &c 2 0 39. Upper bed •» Downs Vein. These 1 6 22. Red rag 10 40. Lower bed/ divide into slates. i 0 Shiver, &c 2 0 Shiver 0 4 Upper rag 3 0 41. Cinder, useless. It is a bed of 23. 12 0 Shiver, &c 2 0 oysters .... } 24. Under rag 16 Stone, not worked 0 4 Shiver, &c 2 0 42. Button; splits into slates . . . 0 7 25. Lead bed 0 4 43. Feather, good . 2 0 26. Shidbed 2 0 44. Cap, used only in backing . . 1 0 27. Shingle, 2 beds 10 45. Flint, ditto 1 0 Shiver, &c 0 6 1 fi 28. Grub 0 8 46. Upper five-bed, sometimes" Shiver, &c 0 3 splits into 5 . . . . a 0 6 29. Roach in 4 beds, good . -\ « 2 0 47. Under five-bed, ditto . . 0 8 30, Grey bed, good . . . ( oe l o 48. White bed, good .... 1 8 31. Thornback, good . . . j 11> 10 49. Tomb-stone, good . . . 0> 0 6 32. Freestone, good . . . J fe 3 0 50. Pudding, inferior. . . . _, 0 9 Shiver, &c 2 0 51. Sheer, used in backing .... 0 8 33. Lias, not worked...... 1 0 0 *i 34. Lias rag 52. Flint, used only in backing . 2 0

Although most of the stony beds are used for some purpose or other, yet the quarrymen distinguish among them four principal groups, which they in the language of workmen call veins. It is these beds only that are worked for exportation, by means of galleries under-ground, which are carried on for a considerable way, at the expense of the individual quarrymen who are the proprietors *. The exact point of junction of the clay and sands which are the equivalents of the Hastings beds, with the Purbeck limestone beds, cannot be seen in Swan- wich Bay, as at this place there is a hollow that is level with the sea; but it may be studied in Warbarrow Bay, , and several places to the west. The bed which appears to be uppermost in the series of the Purbeck limestone is one containing a very large proportion of green earth, the calcareous matter appearing to be derived entirely from the fragments of a bivalve, of from an

* Specimens of all these may be seen in the collection of the Geological Society. Downloaded from http://trn.lyellcollection.org/ at University of Glasgow on June 26, 2015

40 Mr. WEBSTER on the Purbeck and Portland Beds. inch to an inch and a half in length; but during the short time I was on the spot I was not fortunate enough to find perfect specimens. This bed., which is of a thickness that varies from 1 foot to 2 feet or more, passes gradually into that called the , which contains several univalves as well as bivalves; among which, one resembles that of the Petworth marble, and is considered by some as a Paludina, although in general a smaller species of univalve is most abundant*. The cavities of these shells are usually filled up by a compact limestone, and between them is a calcareous matter with much green earth and red oxide of iron. This circumstance occasions some speci­ mens of the marble to have a variegated and pretty appearance; in which it differs from the Petworth, which is usually of a grey aspect. On each side of these beds of marble, which seldom exceed 6 inches in thickness, are frequently layers of fibrous carbonate of lime and also of fibrous sulphate of lime; and considerable masses of the latter have sometimes been found. The Purbeck marble was in ancient times much employed in England for making the slender shafts for the columns in Gothic churches, and in them many fine specimens of this rock may be seen ; but the introduction of foreign marbles has occasioned its use to be discontinued. Occasionally, this marble exhibits no shells, and it is then of a greyish colour, veined with a darker grey. The rest of the Purbeck limestone beds appear to be derived from frag­ ments of shells, which in most instances are sufficiently evident; but the stone is sometimes almost compact. The greatest part of these fragments appear to me to have belonged to small bivalves; but it is extremely difficult to procure such specimens as can enable one to speak with any thing like certainty as to the species. That the univalves in the marble, as well as in the other beds, re­ sembled the Helix vivipara, was observed by Woodward long ago, and I have alluded to this in one of my letters to Sir H. Englefield: but according to my own way of viewing the subject, it would be incorrect to call this a decidedly fresh-water formation, since I think this term ought to be restricted to beds supposed to have been formed in lakes; and I have no doubt but that the Purbeck beds in general contain a mixture of fresh-water with marine shells. I possess some specimens in which are beautiful calcedonic shells, both fresh­ water and marine; and one bed in particular, called Cinder, consists almost entirely of a mass of oysters. Fossil fish, nearly as beautiful as those of Monte Bolca, are not unfrequently found in the Purbeck beds, several of which are in the museum of the Geological Society. Fragments of the bones of the turtle are also of frequent occurrence, and several entire skeletons have been * Sometimes, however, layers are found with univalves as large as those of the Petwcrth marble. Downloaded from http://trn.lyellcollection.org/ at University of Glasgow on June 26, 2015

Mr. WEBSTER on the Pur beck and Portland Beds. 41 found nearly entire. I have also seen fins of the balistes, and teeth of the same species offish as I have mentioned to be found at Hastings. (See p. 35.) As the thickness of all the formations below the chalk diminishes in a very remarkable manner in proceeding from the east end of the Isle of Purbeck westwards, so the beds that we have been considering" partake of this change; for although the quarrymen can point out, in the quarries to the west (as, for instance, in one at Warbarrow), beds which they consider to correspond to the veins which they work at Swanwich, yet the greatest part of the beds to which they give names at the latter place do not exist westwards. At the quarries of Tillywhim, Warbarrow, and Mewp Bay, the junction of the pro­ per Purbeck beds containing fragments of shells, with the beds analogous to them in the upper part of the series in the Isle of Portland, to be afterwards described, may be advantageously examined. I shall next proceed to point out what I have observed chiefly remarkable in the Isle of Portland. The principal quarries in this place were formerly on the north-east side; but these are now almost abandoned, and they procure the stone more conve­ niently on the highest part of the island at the north end. These quarries are not worked by galleries, but are all open to the day. The section of one of the principal quarries on the north end is as follows*. (See Plate VI. fig. 3.) Immediately under the soil, which seldom exceeds a foot in depth, is a series of thin beds, all together about 3 feet thick, called slate by the quarry- men, which split readily into layers from an inch to half an inch in thickness. They consist of limestone of a dull yellowish colour, extremely compact, and entirely without shells (at least I have not seen any in it). In its aspect it con­ siderably resembles those compact varieties of Purbeck stone in which the remains of shells are not visible. These are what I consider to be analogous to the beds in the lower part of the Purbeck limestone, seen at Warbarrow and Mewp Bays above mentioned. Below this is another mass of calcareous stone, considerably softer, and of a lighter colour than the preceding: it is divided into two by a slaty bed, the upper being called aish, and the lower the soft burr. The latter stands upon a bed, about one foot thick, consisting of a dark-brown substance, and contain­ ing much earthy lignite; this bed is very remarkable, and extends all through the north end of the Isle of Portland; I even found some traces of it in the coves at the west end of Purbeck : it is called by the quarrymen the Dirt bed. In it are found considerable numbers of fossil trunks of trees of the dicotyledo-

* Specimens of all these beds I have placed in the collection of the Society. VOL. II. SECOND SERIES. G Downloaded from http://trn.lyellcollection.org/ at University of Glasgow on June 26, 2015

42 Mr. WEBSTER on the Purbeck and Portland Beds. nous class, which are from 1 to 2 feet in diameter. The woody part is siliceous,, and the longitudinal vessels are filled by, and surrounded with, radiated quartz: numerous veins of chalcedony and quartz also pass through these stems, but always following the direction of the concentric and radial structure. In the cavities, and particularly on the outside, there is a small quantity of carbonate of lime; but this is only superficial, and has been a deposit subsequent to the silex. I saw one of these trunks standing erect, and the workmen informed me they were frequently found in this position : its lower part was thickest, and being divided, it gave the idea of the commencement of roots: its upper part penetrated through the soft burr and terminated in the aish. The trunks are always broken off short, seldom exceeding 3 feet in length. This is the fossil wood which is so often brought from the Isle of Portland; and upon the most careful inquiry and examination, I could not discover that it was found in the oolite itself, nor in any other part of the series. In this earthy bed are also many stones evidently water-worn, which I afterwards ascertained to be­ long to the lower part of the Poxtfand series ; one was a dark grey splintery limestone, and another a brown stone slightly oolitic. The bed below this is called the Top Cap, and varies considerably in its structure. Some parts of it are entirely compact; in other places it contains compact parts imbedded in a softer rock; and in others again it is slightly cel­ lular ; but, as far as 1 could discover, contains no fossils. The next bed is catted the School Cap, and is of a very remarkable struc­ ture ; it consists of a compact limestone extremely cellular, the cavities being almost filled with groups of rhomboidal crystals of carbonate of lime. Under the School Cap is

Mr. WEBSTER on the Purbeck and Portland Beds. 43

found in convenient situations. Beneath this,, the quality of the rock changes considerably; it becomes more varied in its texture, and is much mixed with chert. Some parts of it are not oolitic, but grey, and of a splintery fracture ; other parts are brownish and slightly oolitic; while others again are very soft and white, and contain bodies resembling parts of coleopterous insects. This last portion was within 18 inches of the Kimmeridge beds. It is rather to the lower, than to the upper bed, of the good stone of Portland, that I refer the Purbeck Portland, which is generally but slightly oolitic, and contains much calcareous spar: this renders it a harder stone, and fitter for many purposes of building where durability is required. I should state that with the Purbeck Portland is also found another stone not at all oolitic, but gra­ nular, and resembling exactly the stone of Caen in Normandy, which, it is well known, was formerly imported into this country, and much employed in sculpture. I examined also several other quarries at the north end of the Island, where the same beds, and nearly in the same proportion, could be distinguished : but on the south-east side, where quarries are also worked, the beds differ consi­ derably in their subdivisions. The Slate and Cap occur, but not the lignite ; and the Roach, though still considerable, was subdivided into 3 beds; below which the chert was in greater quantity than on the north side. At the south end of the island, from the dip being to the south, the top of the series came within about 20 or 30 feet of the sea, and the lower beds were invisible. There are no quarries in this part. Immediately below this great calcareous series, in the Isle of Portland, are seen very thick beds of bituminous clay and bituminous limestone, resembling those at Kimmeridge which I had before visited: but as it is not my object at present to enter into the consideration of the Kimmeridge strata; I shall only now state that the identification of the beds at the base of Portland Island and at Kimmeridge is, I conceive, as complete as such a subject admits of. Having now taken a rapid view of these two series, viz. the Purbeck and Portland beds, which well deserve a still more particular examination, I cannot help stating that there appears sufficient reason for keeping them separate from each other in grouping the strata. Perhaps no distinction is more remarkable than that depending upon the organic remains derived from fresh-water and marine animals; and although this subject, and the mode of recognising the fossil remains, be extremely difficult and obscure, yet the facts I shall proceed to state, added to so wide a variation in mineralogical character, may perhaps justify my opinion. I have in another place alluded to the analogy between the Purbeck beds G2 Downloaded from http://trn.lyellcollection.org/ at University of Glasgow on June 26, 2015

44- Mr. WEBSTER on the Purbeck and Portland Beds. and the beds in the Weald clay, called, in the , Platten, and in Sussex, Petworth Marble : that the shells in the latter are fresh-water shells, is an opinion which has been advanced by several geologists. The most abun­ dant of the small univalves and bivalves in the Purbeck beds are the same as those in the Hastings rock, which contains scarcely any other shell. Beds of the latter are quarried near Battle Abbey in Sussex, having a very close re­ semblance to some of those in Purbeck, and like them are separated from each other by shale. These and other circumstances may lead us to unite the Pur­ beck beds, the Hastings beds, and the Weald clay into one group; while the same reasons will tend to remove the Purbeck beds from the Portland rock, which contains a suite of marine shells only, as the Turritella, Trigonia, &c. ; added to this, I have no where seen in the Purbeck or Hastings beds any of those oolitic grains of which the Portland almost entirely consists. The chief question is, perhaps, where the line should be drawn between this group and the proper oolite formation; and here, I own, I do not see my way clearly. The beds called the Top Cap have no shells to enable us to decide; but there is something in their general aspect that strongly reminds me of the fresh-water rocks in the basin of Paris, particularly that of Chateau London. Should this resemblance be at a future time found to be owing to similar agencies, the division may be made at the top of the chert, which is evidently connected with the oolite by its containing the same fossils. The Cap of Portland has much more analogy with the lower part of the Purbeck beds than with the oolitic beds; indeed I have seen among the beds of Purbeck some not to be distinguished from the Cap of Portland.