Dr. PiTTON on the Strata below the Chalk. 209 



men call "rubbish " and "slate ", intervene between the lowest of the courses 

 (or "veins") of good stone^ and the top of the Portland formation ; so that 

 the total thickness of the Purbeck formation here is about 275 feet. 



Ft. hi. Ft. In. 



Workable stone, and " rubbish" 99 ■» 



"Cinder" 12 k 124 8 



Workable stone, and " rubbish " 13 8 J 



"Rubbish" and "slate" 150 



Total 274 8 



The high ground between Peverell Point and Durlstone Head is divided by a depression or 

 ravine, on the north of which, in addition to the flexures and contortions seen in all the sections, 

 and well represented in Mr. Webster's plates, the strata are traversed by fissures, produced by 

 upheaving, or subsidence, or both. The effect of these disturbances can be traced by means of 

 the " cinder " bed ; disjointed portions of which are still visible, inclined at different angles, in 

 three or four successive falls, the first throwing down that bed more tlian 100 feet, and others 40, 

 15, and 3 feet. The fissures which separate the disjointed masses are widest at the top, and are 

 filled with fragments of the dislocated strata. The place where these derangements occur is 

 called "the Gulley": on the south of it the strata are much less disturbed, and the " cinder " can 

 be traced almost continuously in its proper situation, till it disappears near the face of the hill, 

 not far from Durlstone Head. 



The groups at the top of the formation called the " Marble " and the " Marble rag ", consist for 

 the greater part of small Paludinae, cemented by carbonate of lime with a very large proportion 

 of green matter. These beds are exposed on the shore at Peverell Point*, and were quarried 

 many years ago at Langton, from which place it is said the pillars in the interior of Salisbury 

 Cathedral were obtained. Another stratum, also containing a large quantity of green matter, but 

 of a conglomerated or coarse sandy texture, is found at the top of the formation in some of the 

 sections westward, — as near Worbarrow Knob, and on the east of Lulworth Cove. It includes a 

 large and thiclc species of Unio ; the shell, as Mr. Webster has remarked, being so abundant as to 

 constitute a very large proportion of the whole mass. 



A characteristic of the upper part of the Purbeck series, both on the coast and in the interior, 

 is the occurrence of seams of fibrous carbonate of lime, frequently more than two inches thick, 

 and either detached within the beds of clay, or adhering to the limestone beds, like the thinner 

 crusts attached to the limestone of the Weald clay. 



The bivalves, principally Cyclades, of which a great part of the Purbeck stone is composed, are 

 not less numerous, and commonly much more distinct, in the slaty clay of the " rubbish" between 

 them ; which cannot be distinguished from the clay with Cyclades (" Shab ") of the limestone pits 

 on the north-west of Battle in Sussex. The " Slate", a coarsely fissile limestone at the lowest 



* A very remarkable appearance, which I am at a loss to explain, was observed by Mr. Bab- 

 bage and myself in 1824, on the back, or top of the Purbeck strata, at that time exposed on the 

 shore between the town of Swanage and Peverell Point. The surface there, which dipped at an 

 angle of 7° or 8° towards the north, was depressed in some places into nearly circular pits or 

 cavities, from 4 to 7 feet in diameter, and about a foot deep in the middle, — as if the beds had 

 been forced in by a violent blow ; the depressed surface being divided by irregular but nearly 

 concentric cracks, which were filled with white sparry carbonate of lime. Three of these 

 depressions were visible ; two of them about 6 feet apart, one of which was 7^ feet, the other 

 about i~ in diameter at the outeredge. A third, about 10 paces to the west of these, was of 

 smaller dimensions. 



2e2 



