222 II. PUMPELLY SECULAR ROCK-DISINTEGRATION. 



" It always occurs directly under the sedimentary rocks, and no similar formation 

 occurs lower down. It is clearly unconformable to the great mass of the schist and 

 gneiss, though precisel}'- like them in its changes. In consideration of all the circum- 

 stances, one cannot resist the temptation to regard it as belonging to a later period." 



Endlich''^ describes contacts between the Potsdam quartzite and crystalline 

 granitic rocks, in the Wind river country, where he finds it often difficult 

 to draw the line between the two. " In certain instances the quartzites and 

 granites blend into each other." 



St. Johnf finds, in the Gros Ventres, the quartzite in the following rela- 

 tion : 



"It is in contact with the unconformable Archean schists, from which it is sepa- 

 rated by a handsome, rose-colored, finely-laminated gneissose lamina, which may be 

 the metamorphosed inferior layer of the quartzite." 



Loew| mentions the following: 



"Occasionally, as for instance between Martin's ranch and Cajon pass, the granite 

 gave rise to the formation of beds of arkose, a rock in which granite debris has been 

 recemented, forming a sort of granitic sandstone resembling to some extent a gran- 

 ite." 



These instances all point to a pre-Cambrian disintegration. The quartzite 

 was formed from the residuary quartz of the disintegrated rock and the 

 feldspar contributed by the feldspar-gruess. 



Reference is made by N. H. Winchell§ to the occurrence in various 

 places about Fort Ridgely, Minnesota, of a substance which is found under 

 the Cretaceous beds " where they overlie the granite," and which passes by 

 "slow changes into the granite," and " seems to be the result of a change 

 in the granite itself" It has some of the characters of steatite and some of 

 kaolin. Professor Winchell adds: 



" It prevails in the Cretaceous areas, and is always present, so far as known, 

 whenever the Cretaceous deposits have preserved it from disruption by the glacial 

 period * * *." 



Upham|| describes the decomposition of the gneiss and granite to a depth 

 of 20 or 30 feet in places, in Brown and Redwood counties, Minnesota, and 

 to at least 10 feet in Renville county. These examples point to a pre-glacial 

 disintegration extending through Mesozoic and Tertiary time which, over the 

 glaciated areas, furnished ready-loosened material for the glacial debris. 



*lUh Ann. Rep. Geol. and Geogr. Surv., 1879, pp. 68, 70, 71. 



t Ibid., p. 411. 



I Geol. and Geogr. Surveys West of the 100th Mer., 1876, Part 3, p. 394. 



g Second Ann. Rep. Geol. and Nat. Hist. Survey of Minnesota, 1874, p. 163. 



il Final Reports of Geol. Survey of Mijinesota, vol. 1, 1884, pp. 570-572; vol. 2, 1888, pp. 190-197. 



