136 EEPORT OF THE 



the existence of animal life The Chemung Group, supposed to 

 be represented by our Marshall Group, has been traced uninter- 

 ruptedly into Ohio, where it becomes almost non-fossiliferous. 

 The Marshall Group is totally isolated from rocks ol 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 Napoleon 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 

 remain in doubt. 



If anything were wanting to show that the geological column 

 in Michigan has been built up as a distinct and independent 

 structure, the existence of the gypseous or Michigan Salt 

 Group, supplies the deficiency. But even further than this, no 

 obvious parallelism has yet been traced between the overlying 

 carboniferous limestone, and the groups of this system further 

 west. The indications already pointed out, however, lead to 

 the conjecture that our limestone was accumulating during sev- 

 eral of the epochs into which geologists have divided this 

 period, though the isolation of our sea has resulted in little 

 correspondence of organic remains. The paucity of rock-pro- 

 ducing materials seems to have continued through the epoch of 

 the coal — our measures not attaining one-twentieth the thick- 

 ness of the same rocks in Ohio. The evidences lead us to the 

 conviction that the Ohio and Michigan coal basins were never 

 continuous, and that the waters did not flow over the separating 

 ridge between the close of the Eelderberg period and the Drift. 

 It cannot be denied, however, that, supposing the carboniferous 

 sea to have been a general one, the remoteness and comparative 

 isolation of the Michigan bay, furnished occasion for great con- 

 trasts in atratigraphical, lithological and palseontological 

 characters. 



One other class of facts must be referred to, which weigh in 



