Mr. J. Cleghorn on the Formation of Rock Basins. 313 



Carentan beds of Normandy. With regard to the denudation of the 

 Weald, Mr. Prestwich suggests that, the anticlinal axis of the Weald 

 having been somewhat raised during the cretaceous period, and the 

 lower tertiaries partly constructed from its debris and gradually 

 distributed over its area, it was again denuded to a considerable 

 extent in the later tertiary period, some island or islands of the 

 lower cretaceous rocks remaining in its area from which for the 

 most part these sandy ferruginous Crag-beds were derived. The 

 great or final elevation and denudation of the Wealden area was 

 necessarily subsequent to the deposition of these pliocene beds, for 

 their outliers, resting on an old flint-drift, occur on the very edge 

 of the upraised chalk-escarpments of the Weald. This elevation 

 being also subsequent in time to the first or Lower Crag period, 

 Mr. Prestwich suggests, that we have here evidence of the physical 

 cause of the distinction of the two Crag periods. The first Crag 

 sea was open to the south, and of considerable extent ; but the last 

 Wealden elevation, cutting off the southern portion, so altered the 

 hydrographical conditions of the period, that a sea open only to the 

 north remained, in which the Red or Upper Crag, with its boreal 

 fauna, was then deposited. 



2. " Notice of the Occurrence of a Malacostracous Crustacean, 

 and of a new Chiton in the Magnesian Limestone of Durham ; with 

 Remarks on some other Permian Fossils." By J. W. Kirkby, Esq. 

 Communicated by T. Davidson, Esq., F.G.S. 



The author has in the course of the last three years met with six 

 imperfect specimens of a minute Crustacean, characterized by two 

 large caudal rings, about nine or ten narrow body rings, and a large 

 cephalic carapace with two prominent hemispherical eyespots, placed 

 far forwards. This animal appears to be the same as that named 

 Trilobites prollematicus by Schlotheim, and Palceocrangon prohlema- 

 ticus by Schauroth. It is not, however, related either to the Trilo- 

 bites or the Crangons ; but is pronounced by Mr. C. S. Bates to be 

 probably Isopodous in its relations ; and more nearly representing 

 the immature than the mature form of recent Isopods. The other 

 fossils described or noticed in this paper are the Chemnitzia Roess- 

 leri. Chiton Howsianus, Lima Permiana, and Hippothoa Voigtiana. 



February 4, 1857. — Colonel Portlock, R.E., President, in the Chair. 



The following communications were read : — 



1. " On the Formation of Rock Basins." By J. Cleghorn, Esq. 

 Communicated by Sir R. I. Murchison, F.G.S. 



In this paper the author referred to the existence of pot-holes on 

 the shore of Caithness, which he had reason to believe were formed 

 in the first place by the grinding action of a loose mass of rock 

 rotated on one spot by the action of the waves, and in the second 

 place by the wearing action of pebbles and boulders washed about 

 within tlie pots or basins by the sea. Mr. Cleghorn referred also to 

 the rock basins of Dartmoor, and the Giants' Pots of Sweden, and 

 stated bis belief that these also had originated in some modifications 



