392 Prof. E. Forbes on the Succesdon of Strata 



value on account of the great number of species ; it is remarkable 

 on account of the new and interesting light it throws on the distri- 

 bution of freshwater creatures during the oohtic period. 



The points at which these observations were made were Lul- 

 worth Cove and the neighbouring bays, Warbarrow Bay, on one 

 side of which, at Meup's Bay, is the clearest and most complete of 

 all the Purbeck sections, Osmington, Upway, and Ridgeway, between 

 Weymouth and Dorchester and Durlestone Bay, near Swanage. 

 Subsequently the base of the Purbecks, as exposed in the great 

 Portland quarries at Swindon, a section which had previously been 

 examined and accurately described by the Rev. J. Brodie, was vi- 

 sited and found to correspond exactly in mineral characters and 

 organic contents, with the beds at the base of the Purbecks in Dor- 

 setshire. From all these localities ample collections were made, in 

 which the author had the constant assistance, for several months, of 

 Mr J. Gapper, of the Geological Survey. 



The results of these researches, whether new, or confirmatory of 

 older observations, may be stated briefly as follows : — 



\st. There is no passage from the Portlands into the Purbecks. 

 The top beds of the Portland stone are purely marine : the lower- 

 most beds of the Purbeck are purely freshwater, containing Cypri- 

 des, and species of Valvata and Limneus. At Meup's Bay these 

 lowest freshwater beds, forming the " Cap," occupy a thickness of a 

 little more than 8 feet, and are surmounted by the great dirt-bed, 

 containing the stools of Cycadacese. A little above the great dirt- 

 bed is another less developed, and a similar one occurs in many 

 places below it. 



Above the highest dirt-bed, the cypridiferous shales, which fol- 

 low, are strangely contorted and broken up in all the sections at 

 the west end of the isle of Purbeck. These are capped by undis- 

 turbed beds full of Cyprides, succeeded by 20 feet or more of shales, 

 calcareous slates, marls, and limestones, with occasionally siliceous 

 bands, all for the most part deposited in brackish water, and filled 

 in many places with JiissocB, of the subgenus Hydrohia^ and a little 

 Cardium, of the subgenus Protocardium. Many of these beds 

 abound in a species of Serpula. closely allied to, if not identical with, 

 Serpula coacervites of the German Purbecks. There are above 30 

 feet of these brackish-water beds at Meup's Bay. They are capped 

 by purely freshwater marls, containing the same species of Cypris, 

 Valvata^ and Limneus, as occur in the lowest bed of the Purbecks. 

 Suddenly, without any disturbance, a change takes place. A 

 very thin band of greenish shales, full of impressions of leaves like 

 those of a large Zostera, and with traces of marine shells, cuts off 

 the freshwater strata. Immediately, however, new freshwater beds 

 succeed, filled in many places with fossils, species of Cypris, Val- 

 vata, Paludina, Planorbis, Limneus, Physa, and Cyclas, all dif- 

 ferent from any he had previously seen in the lower beds. Thick 



