GEOLOGICAL EXPLOHATIOXS AND LITEEATORE— 1S5L 29 



Silurian age, all llu! members of the Azoic system, all the meml)ers of the 

 Volcanic series except the basalts and amygdaloids, and representatives 

 of all the Plutonic rocks. 



The oldest class of igneous products consists of hornblende and felds])ai' rocks 

 and serpentine rocks, and may be regarded as contemporary with the Azoic system. 

 Next in order are the granites and syenites, which are iuterniediate in age between 

 the Azoic and Silurian systems. These are traversed by at least two systems of 

 greenstone dikes, which are anterior to the purely sedimentary deposits. * * * 



Below all the fossiliferous groups of this region there is a class of rocks consist- 

 ing of various crystalline schists, beds of quartz, and saccharoidal marble, more or 

 less metamorphosed, which we denominate the Azoic system. This term was first 

 applied by Murclnson and De Verueuil to designate those crj'stalliue masses which 

 preceded the Paleozoic strata. In it they include not o7ily gneiss but the granitic 

 and plutoiiic rocks by which it has beeu invaded. We adopt the term, but limit its 

 significance to those rocks which were detrital in their origin, and which were sup- 

 posed to have been formed before the dawn of organized existence. (P. .'{.) 



The rocks described by the authors as couiprising- the Azoic system 

 include "gneiss, hornblende, chlorite, talcose, and argillaceous slates, inter- 

 stratified with beds of quartz, saccharoidal marble, and immense deposits 

 of 8})ecular and magnetic oxide of iron." Most of these rocks are regarded 

 as metamorphosed sediments that have been altered by intrusions of trap, 

 basalt, and serpentine, which occur cutting through the sediments as dikes, 

 interleaved with them as sheets, or protruding- through them as bosses. The 

 rocks are contorted. They rarely exhibit the characteristics of sediments, 

 but the evidences of their metamorphic origin are plain, since they become 

 more and more crystalline as the great "lines of igneous outburst" are 

 approached. "Gneiss generally flanks the granite, succeeded by dark 

 masses of hornblende with numerous joints, but obscure lines of bedding, 

 which often graduates into hornblende slate or chlorite slate as we recede 

 from the purely igneous products" (p. 14). In the vicinity of the settlement 

 of Marquette "an epitome of nearly the whole geology of the district" may 

 be observed. Here the authors noted the existence of two quartzite ridges, 

 one on each side of the Carp River. These unite farther westward and 

 form a single ridge that extends beyond Teal Lake. The horizontal Pots- 

 dam sandstone is described as abutting against the quartzite, and the latter 



