AllCriiEAN AND PALEOZOIC. 41 



The thickness of the hiylily inclined and distorted scliists of tlie 

 Archa'iin is practically impossible to estimate with any de<^ree of accu- 

 racy. In their present greatly denuded and metamorpliosed condition one 

 cannot determine whether they arc the remnants of several great anticlinal 

 folds among themselves, or whether they are the broken strata of one vast 

 fold, though the latter, judged from a study of the nature of the rocks, 

 seems the more probable structure. In that case the total thickness of the 

 Archa^.an strata must be more than one hundred thousand feet. 



The study of these rocks has shown them to bo readily divisible into 

 two groups — 1st, a series of coarsely crystalline micaceous schists, and, 2d, 

 a series of very fine micaceous clay slates — the latter of which in structure 

 and associated minerals bears a close resemblance to the Huronian in 

 the typical localities of its development around the Great Lakes. The 

 lithological structure of the metamorphic rocks has been examined criti- 

 cally, both in hand specimens and by a microscopic study of thin sections, 

 by Mr. Caswell, the results of whose labors will be found in the chapter 

 on Petrography. 



Associated with the schists in the southern portions of the Hills are 

 immense masses and peaks of highly feldspathic granite, culminating in 

 the region of Harney Peak ; and on the outskirts of the same district are 

 many smaller masses of the same material. So far as their structure was 

 made out, each of the bodies has a lenticular shape, and is intercalated 

 among the strata of the schist. All the observed phenomena of the occur- 

 rence ot the granitic masses have been weighed and studied with care, 

 and the conclusion that they are of intrusive origin is discussed at length 

 in the section devoted to them. 



In the series of the Paleozoic rocks on the opposite page the facts that 

 will first attract the attention of a student of the geology of the eastern 

 portion of our continent are, 1 st, that the great Silurian system is represented 

 only by a sandrock about 250 feet in thickness, referable to the Potsdam 

 period, and 2d, ^hat the entire Devonian system is absent. The study of 

 the Potsdam was not surpassed in interest by that of any other formation 

 of the country. It was in this region that the Silurian in the Far West was 

 first recognized, and the collections made the last summer have, by the study 



