226 Geological Society, 



stroyed, and its ruins will not be distinguishabe from the 

 other bowlders with which the tops of all the hills in this 

 vicinity are overspread. 



The third drawing rcjiresents the Vixen Tor on Dartmoor, 

 Almost all the granite of Cornwall and Devon, like that 

 of the Land's End, is divided by fissures into masses more 

 or less approaching a cubical form. If a rock of this kind 

 nearly level with the snrlace of the soil is exanuned, the 

 fissures will be found to be a mere mathematical plane, and 

 the angles formed by the intersection will be sharp and 

 perfect. If we then turn our attention to granites which 

 from their greater eKvation above the soil appear to have 

 been longer exoosed to air and weather, we may observe a 

 gentle rounding of the angles such as is exhibited in the 

 Vixen for. 



By degrees the fissures become wider; and the blocks, which 

 were originally prisn)atic, acquire an irregular curvilinear 

 boundary resembling those whicli form the Cheese-wring, 



If ihe centre of gravity chances to be high, and far re- 

 moved from the perpendicular of its fulcrum, the stone falls 

 from its support, and becomes rounder by the progress of 

 decomposition, till it assumes one of the various spheroidal 

 figures which the granite bowlder so often exhibit. 



These fissures, and the roundeaform which the cubical 

 blocks acquire by decomposition. Dr. M. is inclined to at- 

 tribute to the original structure of the rock. In this, as in 

 basalt, crystallization appears to have begun in distinct and 

 more or less distant points, from each of which it proceeded 

 forming thick concentric laniellae, till at length the exterior 

 shells of adjacent concretions came in contact, but were in-r 

 capable of mutual penetration. The outer lamellas are the 

 least hard and dense, and therefore yield the easiest to the 

 various causes whicb occasion the disintegration of rocks. 



March \Q. Mr. W^ebster exhibited specimens of the rocks 

 containing fresh-water shells, recently discovered by him in 

 the Isle of Wight. 



A paper on the rocks at Clovtlly in Devonshire, with 

 illustrat ve drawings by the Rev. J. J. Convbeare, was read, 

 and thanks were voted for the same. 



The fishing to \n of Clovelly is situated in a narrow 

 ravine, on the N. coast of Devon, about 22 miles W. of 

 IlfracQmbe. The shore is precipitous, being formed of cliflTs 

 about 130 or 140 feet in height, intersected by narrow 

 alternations of grauwaeke and grauwacke slate, curved and 

 contorted in the most capricious way imaginable. No 

 organic remains were observed in them, nor any foreign 



minerals. 



