82 The Eozoic Bocks of North America. 



IV. The Eozoic Eocks of North America, * 



By T. Sterry Hunt, LL.D., F.R.S. 



According to the author there is found among the pre-Cani- 

 brian strata of North America an invariable succession of 

 crystalline stratified rocks, which have been by him divided into* 

 several great groups, the constituents of which become progres- 

 sively less massive and less crystalline until we reach the sediments 

 of paleozoic time, of which the Cambrian is regarded as the 

 basal member. Since all of these pre-Cambrian rocks, with the 

 exception, perhaps, of the lowest or fundamental gneiss, present 

 evidences, direct or indirect, of the existence of organic life at 

 the time of their deposition, it seems proper to include them 

 under the general title of Eozoic, proposed by Sir J. W. Dawson. 

 That of Archaean, employed by some geologists to designate these 

 pre-Cambrian rocks, appears too indefinite in its signification, and, 

 moreover, is not in accordance with the nomenclature generally 

 adopted for the great divisions succeeding. These Eozoic rocks 

 include both the Primitive and the Transition divisions of Werner. 

 We distinguish at the base of the Eozoic series a massive and 

 essentially granitoid gneiss. To this fundamental rock, some- 

 times called the Ottawa gneiss, and of unknown thickness, suc- 

 ceeds what has been named in Canada the Grenville gneissic 

 series, made up in great part of gneiss somewhat similar to that 

 last mentioned, with intercalations of hornblendic gneiss, of quart- 

 zite, of pyroxenite, of serpentine, of magnetite, and of crystalline 

 limestones, the latter often magnesian, occasionally graphitic, and 

 sometimes attaining thicknesses of a thousand feet or more. This 

 Grenville series, the strata of which are generally highly inclined, 

 has an aggregate volume of not less than 15,000 or 20,000 feet, 

 and appears to rest unconformably upon the fundamental or Ottawa 

 gneiss. This gneissic series, with its intercalated limestones, 

 some of which contain Eozoon canadense, was the typical Lau- 

 rentian of Logan and Hunt, named by them in 1854, with which 

 they included, at that time, however, not only the underlying 

 fundamental gneiss, but an upper granitoid and gneissoid series, 



* Abstract of a paper read before the British Association for the Advance- 

 ment ot Science, Montreal, Sept. 1, 1884. 



