507 



ticipated by Messrs. Briggs, Lesley, and Rand, and by Dr. 

 R. E. Rogers. 



Mr. Blake based his arguments on such facts as those 

 which he had observed, in examining the so-called " Grave- 

 stone " slates of California, and pebbly conglomerates of 

 Arizona, in which, he alleged, all the quartz pebbles lying 

 in a mica slate matrix were evidently elongated in one 

 direction. lie did not agree with the late Dr. Hitchcock in 

 calling in the intervention of heat and moisture to account 

 for the elongation of the Vermont and Rhode Island pebbles 

 in conglomerate ; for Kirkaldy's plates were of cold steel. 

 Physicists must accept as a law the plasticity of all matter, 

 even in a dry and cold state, under sufficient!}' exacting con- 

 ditions of pressure and time. 



Dr. Rogers considered such an hypothesis very hard to 

 entertain. He did not believe the alleged fact of the elonga- 

 tion of pebbles in conglomerate rocks. The shape of the 

 Newport pebbles was certainly given to them by attrition 

 on the sea beach before the consolidation of the deposits. 

 The elongation of a quartz-pebble in a softer matrix is im- 

 possible, because the power exerted can have no firm hold on 

 the pebble, which ma}' change its place in the yielding 

 matrix, but without changing its shape. A conglomerate 

 must be homogeneous like a mass of steel to exhibit the 

 effects portrayed by Kirkaldy, and no such conglomerate 

 exists. 



Mr. Lesley agreed with Dr. Rogers in this opinion ; but 

 adduced such facts as the sigmoid flexures in gneiss and 

 mica slate rocks, to show that quartz string-veins are sharply 

 bent without fracture, proving the plasticity of quartz. 

 Sometimes these zigzags are so numerous and the plications 

 so sharp, that, if pulled out straight, an unbroken quartz 

 vein would extend hundreds of feet. He said that once, in 

 company with Mr. Edouard Desor, he had found upon the 

 outcrop of the coal measure conglomerate the impression of 



