682 A. C. LANE AND A. E. SEAMAN 
The Laurentian-Keewatin.—These terms we take to have the 
same stratigraphic meaning, since the relations are of intrusive con- 
tact—the former being applied where the areas are largely felds- 
pathic or granitic and light colored, the other when they are mainly 
of basic rocks, or klastic in a good many cases, of the nature of vol- 
canic tuff (the ‘Greenstone schist” or ‘‘Mareniscan”’). In Michigan 
at least there is comparatively little if any of the commoner types of 
sediment and limestones, either in their altered or unaltered condi- 
tion. Many of the schists which are fine grained and slaty enough 
to pass for altered sediments prove really to be altered felsites, or 
volcanic ash, or something of that kind. ‘There is, however, a little 
iron-ore-bearing jaspilitic chert and rarely genuine black slates. 
Early conditions of erosion before there was any land vegetation 
would have led to the formation of arkoses and tuffs, and there may 
have been less ocean of a very different chemical character, account- 
ing for the scarcity of ordinary types of sediment. Generally speak- 
ing these rocks are more or less saturated with granitic matter, either 
in fine-grained aplitic or coarse-grained pegmatitic veins, but the 
whole rock may be a gneissoid granite. These gneissoid granite 
bosses generally occupy anticlinal areas, and are the typical Lauren- 
tian. Around Marquette there seems to be a compound  synclinal 
with argillites in the upper part and some ill-defined iron-bearing 
beds which may correspond to the Vermilion Range below, and a 
great deal of “Greenstone schists.””? These latter consist of horn- 
blende and chlorite schist and amphibolites, including rarely “ellip- 
soidal greenstones,”’ and sericite schists altered from felsites through- 
out, but especially in the lower parts next to the granitic anticlinal 
bosses. The total thickness one can estimate not less than 1,000 or 
more than 5,000 feet. 
By Bigsby, Maclure, etc., this formation was grouped as gneiss or 
granite with the hornblende slate under the term primitive; by 
Houghton and his assistants as syenite, granite, gneiss under the 
term primary; by Logan as Laurentian; by Foster and Whitney as 
granite rocks of the Azoic period; by A. Winchell as granitic rock 
of the Plutonic group; by Credner as the Lower Laurentian of the 
Eozoic; by Brooks and Pumpelly and many others following them 
as Laurentian; by Rominger as Huronian; by Van Hise and the 
