254 ANNUAL OF SCIENTIFIC DISCOVERY. 



oceans. Under such conditions, we ought not to look for any types of 

 animal or vegetable life. 



Many eminent geologists maintain that the lowest stratified rocks 

 are but portions of the Silurian system, and that, from long-continued 

 exposure to heat, the lines of stratification have become obscure, and 

 all traces of organic remains obliterated. Our observations in this 

 district (they remarked) have led us to a different conclusion. If the 

 Potsdam sandstone rest at the base of the Palaeozoic series ; if from that 

 epoch we are to date the dawn of organized existence, there is, in this 

 district, a class of rocks, detrital in origin, interposed between the 

 lower Silurian system and the granite ; rocks distinct in character, 

 unconformable in dip, and destitute of organic remains. If we found 

 these crystalline schists, and beds of quartz, and saccharoidal marble, 

 graduating into clay-slates, sandstones, and limestones, as we receded 

 from the lines of igneous outbursts, and enveloping the remains of 

 plants and animals, we would be led to a different conclusion ; but so 

 far from it, the evidence is ample that the base of the Silurian system 

 reposes upon their upturned edges, and that the causes by which the 

 metamorphism of the former was effected had ceased to operate before 

 the deposition of the latter. Between the two systems there is a clear 

 and well-defined line of demarcation. It forms one of those great 

 epochs in the history of the earth, where the geologist can pause, and 

 satisfy himself of the correctness of his conclusions. On the one hand 

 he sees evidence of intense and long-continued igneous agency, and, 

 on the other, of comparative tranquillity and repose. 



The Azoic rocks occupy an almost continuous belt along the northern 

 shore of Lake Superior, subject, however, to occasional interrup- 

 tions, and have thence been traced, by Logan and Bayfield, to the 

 coast of Labrador, forming the axis between Hudson's Bay and the 

 Valley of the St. Lawrence. 



They are also extensively developed on the southern shore, forming 

 the water-shed between the respective river-systems of Lake Superior, 

 Lake Michigan, and the Mississippi. Wherever the junction between 

 the Azoic and Silurian systems has been observed, the one is found to 

 repose, unconformably, on the other. As the Silurian system on Lake 

 Superior is celebrated for its deposits of copper, so the Azoic system is 

 equally interesting from its ores of iron. Indeed, at some points on 

 the lake, the iron in the form of a nearly pure oxide forms entire 

 mountains ; one on Carp river being 1,067 feet above the level of the 

 lake. From the detailed explorations of Mr. Mersch, communicated 

 to Messrs. Foster and Whitney, they had no doubt that the Missouri 

 iron region belonged to the same system of upheaval, and occupied the 

 same relation to the Silurian system. They also stated that the mag- 

 netic ores of Sweden, associated with gneiss, belonged to the same 

 epoch. The same was true with regard to the ores of the Champlain 

 region of New York. As to the thickness of the Azoic system, it was 

 impossible to form a correct idea. It might be 20,000, or 50,000, or 

 100,000 feet. If we were to adopt the usual method of measuring 

 across the basset edges of the strata, it would give us a thickness 

 greater than that of the whole fossiliferous series, from the base of the 



