SCIENTIFIC NEWS, 307 



tions, with a marine formation between them. The lowest 

 freshwater formation consists of beds of sand and marl, 

 with numerous fragments of the Limnea of Lamarck, and of 

 two species of Planorbis : the interposed marine deposit is of 

 blue clay with Venuses, oysters, and various turbinated shells : 

 and the upper freshwater formation consists of a calcareous 

 rock, inclosing numerous and very fine specimens of the Lim- 

 nea and Planorbis. Some of this last stratum is very friable,' 

 being only marl ; other parts are extremely hard, and the up- 

 permost part of it has a porcelaneous character. Mr. W. has 

 not discovered any bed of gypsum, nor silicous concretions. 



A paper by Dr. Mac Culloch on the Granite Tors of Devon- 

 shire and Cornwall, accompanied by three drawings, was read, 

 and thanks were voted for the same. 



The object of this communication is to show the process 

 followed by nature in the destruction of the Granite rocks of 

 Cornwall and Devonshire, and to explain how this process is 

 dependent upon a particular mode of aggregation of the mate- 

 rials of which this lock consists, and which can be satisfactorily 

 observed only during the progress of its decomposition. 



Dr. M. first treats of the Granite which forms that promon- 

 tary near the Land's-end on which the Loggan rock is situated. 

 Its length is about 200 yards, and the entire rock of which it 

 is composed is traversed by numerous vertical and horizontal 

 fissures, thus dividing the mass into a multitude of cubical and 

 prismatic blocks. The Loggan rock itself, which is the sub- 

 ject of one of the drawings, appears to be one of these blocks 

 in a state of decay, but still occupying its original site. Its 

 general figure is irregularly prismatic and four-sided, with a 

 protuberance on the lower surface on which it is poised. The 

 breadth of the apparent surface of contact between this pro- 

 tuberance and the rock that it rests upon, is about a foot and a 

 half, but its figure being cylindrical, and not spheroidal, the 

 motion of the stone is limited to a vibration in one direction. 

 The utmost force of three persons (according to Mr. M.'s trials) 

 is only capable of making its exterior edge describe an arc, the 

 chord of* which, at six feet distance from the centre of motion, 

 is three quarters of an inch. A force of a very few pounds, 

 however, is sufficient to begin and maintain a very visible de- 

 gree of vibration. Even the wind, when blowing on its ex- x 



posed 



