558 Mr FISHER, ON THE PURBECK STRATA OF DORSETSHIRE. 



all the quarries are in the lower. The great mass of the stone-beds is due to animal secretion, 

 so that, somewhat paradoxically, we should have the greater deposit where the water was clearer, 

 and vice versa; and this accounts for the upper portion of the lower Purbecks, contrary to the 

 general condition of that part of the series, being more developed at Durlstone, because at both 

 Durlstone and Ridgway these beds (which are in fact the insect beds) are a sedimentary deposit 

 from water turbid with calcareous mud. In other words, where the shelly and cypris beds 

 are thinner there the sedimentary will be thicker. 



It is difficult to obtain a clear notion of the thickness of the upper division at Swavage, 

 on account of the very disturbed position of that portion of the series, and the want of a 

 deep section ; but the general impression conveyed to the mind is that of a series of bands of 

 clay and stone, about 30 feet thick, with very little sand or drift, the clays being charged 

 with cypris shales, and the stone bands with Unios and Paludinas. Mr Austen has given a 

 detached section of them at page 9 of his tract*, including about eight feet, but not joining 

 on to any general section. This however is evident — that they are far less sandy, and 

 contain far less drift, than the same beds at Ridgway. Here too there is a thick band 

 of Paludina marble. At Ridgway it is very inadequately rather hinted at than represented, 

 while the shells are individually very small, the sandy bottom not having suited them so well. 

 At Mupes bay this part of the series bears a close resemblance to its development at 

 Durlstone, but is not so thick. The marble beds are also thinner. There is, however, a 

 greater variety of species of Cypris here than at Durlstone bay. 



At Ridgway these upper beds are on the whole much coarser and more sandy in their 

 texture, and contain a larger proportion of the spoils of the land, in the shape of lignite 

 and plants, &c. There are, however, a few beds of fine-grained marls, and some clays 

 with Cyprides, but on the whole there is evidence of a greater transporting power in the 

 water than in the more eastern localities of our district ; and also (what is generally 

 observable at Ridgway) of a more variable and frequently changing condition of it. The 

 best specimens of Unios have been obtained from this locality. I have not been able to 

 obtain any insect remains from the Upper Purbecks. 



The middle division of the Purbecks is very different in its lithological character from that 

 which we have just considered. The ordinary condition of its limestone beds is that of a congeries 

 of bivalve shells (the paving of Nevilles Court at Trinity College, and of the Hall of Pembroke 

 College is from these beds); and they shew several alternations of fresh, brackish, and salt-water 

 types. Comparing these beds at the same three localities at which we have before compared 

 the upper division — at Durlstone the middle beds attain the greatest thickness, and these 

 form the most important division of the whole Purbeck series. In them are situated the 

 extensive quarries for which the so-called Island has long been famous; and though the 

 chief supply of paving for the London market has now fallen into the hands of Yorkshire 

 stone-merchants, large quantities of this stone are still raised for the supply of Portsmouth 

 and other places in the south. The first bed of stone is that to which Professor Forbes 



' A Guide to the Geology of the Isle of Purbeck. W. Shipp, Blandford, 1852. 



