Geological Society. 225 



parts arc extremely hard, and tlie uppermost part af it has 

 a porcelainous character. Mr. VV. has not discovered any 

 bed of gypsum, nor sihceous concretion*. 



A paper bv Dr. MacCulioch, on the granite Tors of De- 

 vonshire and Cornwall, acconipanied by three drawings, 

 was read, and thanks \\cre voted tor the same. 



The object of this communication is to show the process 

 followed by nature in the dtst ruction of the cranite rocks 

 of Cornwall and Devoubhire, and to explain how this pro- 

 cess is dependent upon a particular mode of aggrc^'ation of 

 the materials of which this rock consists, and which can 

 be satisfactririly observed only during the progress of its 

 decomjiosUion. 



Dr. M. first treats of the granite which forms that pro- 

 moniory near the Land's End on wb.ich the Loggan rock is 

 situated. Its length is about 200 yards, and the entire rock 

 of which it is composed is traversed by uun.erous vertical 

 and horizontal fissures, thus dividing the mass into a mul- 

 titude of cubical and prismatic blocks. The Loggan rock 

 itself, which is the suliject of one of the drawings, appears 

 to be one of these blocks in a slate of decay, but still oc- 

 cupying its original site. Its general figure is irregularly 

 prismatic and four-sidc^.^ with a prot\iberance on the lower 

 surface on which it is poised. The breadth of the appa- 

 rent surface of contact between this prcjtnberance and the 

 rock that it rests upon, is about a foot and a half; but its 

 figure being cyhndrical, and not spheroidal, the motion of 

 the stone is limited lo a vibration in one direction. The 

 utmost force of three persons (according to Dr. M.'s 

 trials) is only capable of makins 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 main- 

 tain a very visible degree of vibration : even the wind when 

 blowing on its exposed western surface produces this eflect 

 very sensibly. Its weight, as estimated from iis dimensions 

 and the specific gravity of granite, appears to be 65 tons. 



The subject of the second drawing is the tlheese- wring, 

 whirh occupies the highest ridge of a hill to the north of 

 I.,iskeard. It is an irregular colnnni ubout 1 ,5 feet liigh, 

 composeii of five stones, the uj-per ones of which are so 

 niuch larger than the re«t as to i.verhang the base on all 

 sides. The angles and external borders of these stones are 

 considerably rounded by the cflect c)f decomposition : and 

 there is no doubl that in process. of time this disintegration 

 will proceed so far thai the baUnict of the pile will be de- 

 Vol. 41.No. 17y. ilfa/'c7i lbl3. P slroyed, 



