274 GEOGNOSY OF THE APPALACHIANS. [XIII. 



the chiastolite-schists of Brittany at the very base of the tran- 

 sition sediments, marking the summit of the crystalline schists.* 



With regard to the crystalline schists of Lakes Huron and 

 Superior, to which the name of the Huronian system has been 

 given, the observations of all who have studied the region 

 concur in placing them unconformably beneath the sediments 

 which are supposed to represent the base of the New York 

 system ; while, on the other hand, they rest unconformably on 

 the Laurentian gneiss, fragments of which are included in the 

 Huronian conglomerates. The gneissic series of the Green 

 Mountains had, however, as we have seen, been, since 1841, 

 regarded, by the brothers Rogers, Mather, Hall, Hitchcock, 

 Adams, Logan, myself, and others, as Lower Silurian (Cam- 

 brian of Sedgwick). Eaton and Emmons had alone claimed 

 for it a pre-Cambrian age, until, in 1862, Macfarlane ventured 

 to unite it with the Huronian system, and to identify both with 

 the crystalline schists of a similar age in Norway. Later ob- 

 servations in Michigan justify still further this comparison; for 

 not only the more schistose beds of the Green Mountain series, 

 but even the mica-schists of the third or White Mountain 

 series, with staurolite and garnet, are represented in Michigan, 

 as appears by the recent collections of Major Brooks of the 

 Geological Survey of Michigan, kindly placed in my hands for 

 examination. He informs me that these latter schists are the 

 highest of the crystalline strata in the northern peninsula. 

 (Ante, page 18.) 



To the north of Lake Superior, as I have already shown 

 elsewhere, the schists of this third series, which, as early as 

 1861, I compared to those of the Appalachians, are widely 

 spread ; while in Hastings County, forty miles north of Lake 

 Ontario, rocks having the mineralogical and lithological charac- 

 ters both of the second and third series are found resting on 

 the first or Laurentian series, the three apparently unconfonn- 

 able, and all in turn overlaid by horizontal Trenton limestone, t 



We have shown, that in Pennsylvania, while some of these 



* Bull. Soc. Geol. de Fr. (2), XVIII. 664. 



f American Journal of Science (2), XXXI. 395, and L. 85. 



