76 REVIEWS — THE GEOLOGICAL SURVEY OF MICHIGAN. 



are tliose of the Drift or Quaternary Period. Before turning our 

 attention to these, we extract from the Report a few observations of 

 much interest in reference to the geological relations of the groups 

 already alluded to : — 



" Many interesting considerations present themselves on a general review of 

 the geology of the peninsula. From the Lake Superior Sandstone to the close of 

 the Helderberg period, our state seems to have had a common history vrith Canada 

 West, and the States on both sides of us. The same groups of rocks are traced 

 uninterruptedly from New York across the peninsula of Canada to Michigan 

 and even to the Mississippi river, preserving throughout that whole extent as 

 great a degree of palceontological identity as could be expected of faunas stretch- 

 ing over so many degrees of the earth's surface. It is true, as has been long 

 since shewn by Prof. Hall, that nearly every member of the Silurian and lower 

 Devonian system, thins gradually in its westward prolongation, loses somewhat 

 of its arenaceous or argillaceous character, and becomes at the west much more 

 calcareous — changes which have generally been regarded as proving the origin 

 of the materials of those groups to have been at the east. It is interesting to 

 observe, however, notwithstanding this westward attenuation, how completely 

 we are able to recognize all the essential features of the New York System.in 

 our own State. 



" From the close of the Helderberg period, on the contrary, Michigan has had 

 a history to some extent peculiar. The rocks of the Hamilton group can indeed 

 be traced almost continuously from New York into our own State, but the 

 palseontological characters are found materially changed, and the strata are 

 more argillaceous. The Portage Group, of New York, supposing it to be repre- 

 . seated by our Huron group, has received great accessions of argillaceous mat- 

 ter, and seems to have been deposited under circumstances more unfavourable to 

 the existence of animal life. The Chemung Group, supposed to be represented 

 by our Marshall Group, has been traced uninterruptedly into Ohio, where it be- 

 comes almost non-fossiliferous. The Marshall Group is totally isolated from 

 rocks of the same age anywhere beyond the limits of our peninsula ; and though 

 the sandstones bear some physical resemblance to those of the Chemung Group 

 of Ohio and New York, our formation contains little or no argillaceous matter ; 

 its fauna is remarkably rich, and its species are nearly all peculiar. The Napo- 

 leon Group, if correctly separated from the Marshall Group, has no distinct 

 equivalent in surrounding States ; and its entire destitution of organic remains 

 will cause its true geological relations to remaia in doubt. 



" If anything were wanting to show that the geological column in Michigan haa 

 been built up as a distinct and independent structure, the existence of the gyp- 

 seous or Michigan Salt Group, supplies the deficiency. But even further than 

 this, no obvious parallelism has yet been traced between the overlying carboni- 

 ferous limestone, and the groups of this system further west. The indications 

 already pointed out, however, lead to the conjecture that our limestone was ac- 

 cumulating during several of the epochs into which geologists have divided this 



