556 



Mr FISHER, ON THE PURBECK STRATA OF DORSETSHIRE. 



I am not however inclined to consider the Purbecks as a portion of the Wealden series, 

 though in immediate sequence; and I am strengthened in this view by the fact of never 

 having seen any of the bones of the gigantic Saurians of the Weald from any part of the 

 Purbeck beds*. 



It is evident that in the Dorsetshire district both the Purbecks and the Wealden beds are 

 quite unconformable to the Cretaceous system, which not only overlaps them gradually, as it 

 does all the Oolites in its extension westward, but also covers them very suddenly at some 

 spots. At Osmington Mills, near Weymouth, we have within less than a square mile the 

 upper Green sand in contact, 1st, with the Hastings sand, 2dly, with the Purbecks, 3dly, 

 with the Kimmeridge clay. And at White Nose Cliff' this very instructive section is exposed, 

 and with a telescope may be observed from Weymouth. 



(a) Chalk. 



(6) Green sand. 



(c) Gait. Fig - (1) - 



(d) Purbeck. 



(e) Portland stone. 

 (/) Portland sand. 

 (g) Kimmeridge clay. 



The late Dr Mantell states in his Wonders of Geology, that he has never been able to 

 obtain pebbles of any but the smallest size from the Wealden strata of Sussex. I can affirm 

 the same with respect to the Isle of Wight, and in Swanage bay there is very little that can be 

 called coarse drift, but more than in the Island, When, however, we reach Worbarrow bay, 

 a few miles westward, there is much coarse drift, sometimes assuming even the character of a 

 fine conglomerate, and at Lulworth I have found pebbles as large as a pigeon's egg, with coarse 

 grains of mica. The same coarse character of the detritus may be remarked at Ridgway-hill ; 

 and it is curious to observe how suddenly the coarse drift is frequently superimposed upon 

 a fine clay or sand, and then gradually diminishes in coarseness until it passes again into a fine 

 sand ; thus relating as plainly as if it were written the sudden swelling of the ancient torrent, 

 and its then gradually subsiding into its normal state. The larger grains consist of quartz, 

 and the pebbles when not of quartz seem always referable to crystalline rocks. There is 

 also much micaj-. Yet the abundance of iron has sometimes led me to speculate upon the 

 new red sandstone of Devonshire as the most suitable deposit to have furnished the requisite 

 materials; for that deposit abounds in layers of waterworn pebbles from the older rocks. 



The difference thus observable between the state of the detritus in the Hastings sands 

 of Sussex, Hants, and Dorset, seems to lead to the conclusion, that the stream flowed from 

 the west towards the east. This supposition is also favoured by the rapid increase of 

 thickness in the deposit as we proceed eastward. In the Isle of Wight, the lowest beds 



* Perhaps, however, the quieter waters could not bring 

 them down. 



f The firmness of sea-sand seems due to mica. Contrast 



the sands at Bournmouth derived from the lower Bag- 

 shots with those at Brixton or Swanage from the Hastings 

 series. 



