218 



R. PUMPELLY — SECULAR ROCK-DISINTEGRATION. 



trition and a very fine angular sand ; ^^ a conglomerate resulting from this 

 process would have granite pebbles, not pebbles of pure quartz. But, in 

 a mantle of disintegrated granite, the quartz is loose and ready to wear 

 down by attrition to the smallest size that the local conditions of velocity 

 will admit of; and the process is facilitated by fracture along the planes 

 that separate the individuals of the quartz aggregates, as suggested by 

 Daubree. 



The evidence of successive periods of ancient secular disintegration is 

 necessary to support an hypothesis which presupposes their existence in ex- 

 plaining the apparently conformable transition between schists of widely 

 different ages. 



For early pre-Cambrian or pre-Algonkian time we have this evidence 

 very clearly in the conglomerate forming the base of the graphitic limestone 

 along the eastern edge of the Adirondacks. This limestone was found by Mr. 

 Walcott, at Fort Ann, to carry fragments of older crystalline rocks. Going 

 from this point to Westport, New York, Mr. Walcott, Professor Van Hise 

 and myself found the bottom of this limestone everywhere charged with 

 pebbles and bowlders of the older rocks and with great quantities of large 

 fragments of the triclinic feldspar of the underlying hypersthene-granite. 

 The appearances are that the limestone ushered in the period to which it 

 belonged, and without any violent breaching action of the wat^r. There is 

 little evidence of attrition in the fragments, which appear to be cores, some 

 only partially divested of their concentric altered layers. In some cases, 

 these old cores are surrounded by what appears to have been a disintegrated 

 shell, between which and the core, calcite has crystallized and new minerals 

 have formed. This limestone is covered by a great thickness of garnetiferous 

 gneiss, over which the fossiliferous Potsdam sandstone lies unconformably, as 

 was pointed out by Mr. Walcott. 



In the central ridge of the Green mountains we have one or more lime- 

 stones, which have been assigned to the "Azoic " by President Hitchcock, 

 lying below a series of highly crystalline gneisses, which, like the limestone, 

 are represented in the pebbles of the basal conglomerate of the Cambrian. 



These limestoneSj like that of the Adirondacks, rest generally on coarsely 

 crystalline granitic or gneissoid rocks, without any intervening clastic sedi- 

 ments. But the underh'^ing rock has almost everywhere the appearance of 

 having been originally a coarse granite which has undergone a crushing 

 action. The feldspar grades from large, one-inch crystals with wavy cleav- 

 age, into coarse and fine granulated aggregates; the quartz also is granu- 

 lated. Moreover, some of the large crystals of feldspar, which have the 



*This is clearly shown by the experiments of Daubree ("Etudes Synthetique," 1879, p. 253). 

 Daubr6e also states that the ordinary sand of sandstones cannot be a product of attrition under 

 torrents or wave actipn, and that sonie other source for them must be sought. The two sources 

 that occur to him are glacial grinding, which produces coarse and fine grains, and disintegration 

 under weathering. 



