REVIEWS — THE GEOLOGICAL SURVEY OF MICHIGAN. 11 



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

 organic remains. Tlie paucity of rock-producing materials seems to have con- 

 tinued through the epoch of the coal — our measures not attaining one-twentieth 

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

 that the Onio and Michigan coal basins were never continuous, and that the waters 

 did not flow over the separating ridge between the close of the Helderberg 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 contrasts in stratigraphical, litho- 

 logical and palaeontological characters. 



" One other class of facts must be referred to, which weigh in the same direc- 

 tion. They constitute evidence that the materials for our upper Devonian and 

 carboniferous rocks have been derived from the north. The Helderberg lime- 

 stones are 350 feet thick at Mackinac, and not more than 60 feet thick in Monroe 

 county. The Hamilton Group, so well developed in Thunder and Little Tra- 

 verse Bays, is not recognized in the southern part of the State. The Huron 

 Group, with its gritstones and flagstones at Pt. aux Barques, contains only two 

 strata of flagstone at Grand Rapids. The conglomerate at the base of the Mar- 

 shall Group, at Pt. aux Barques, is recognized at none of the Southern outcrops. 

 The pebbles scattered through the Marshall and Napoleon Groups in Huron 

 county, are entirely wanting in Jackson and Calhoun counties ; while, on the 

 contrary, extensive patches of the Marshall sandstone are found finely cemented 

 by calcareous matter at Battle Creek, Jonesville and other southern points. 



" One other remark is suggested by this review of our rocks. The geology of 

 Michigan discloses little connection between the Carboniferous Limestone and 

 the Coal Measures ; while the transition to Devonian rocks is imperceptible. I 

 see no reason for drawing the broad lines which separate great systems, between 

 the Marshall and Napoleon groups, or between the Napoleon group and the Car- 

 boniferous limestone. On the contrary, I see this limestone characterized by a 

 peculiar, persistent, marine fauna, while the Parma Sandstone, the Coal Measures 

 and the Woodville Sandstone, were accumulated in shallow waters near shores, 

 or even in marshes ; and are characterized, from bottom to top, by evidences of 

 the proximity and abundance of terrestrial vegetation. These contrasts hold 

 throughout the country, and in all countries. Whatever marine remains are 

 found in the coal measures, belong to species distinct from those in the Carboni- 

 ferous Limestone ; and if the generic distinctions are not complete, the organic 

 facies of one is vegetable and terrestrial ; that of the other, animal and marine. 

 Downward the types of the lower Carboniferous rocks descend into the upper 

 Devonian — some carboniferous species, and numerous carboniferous types, even 

 reachmg the Hamilton group. Observations in Michigan suggest rather to draw 

 the broad systematic lines below the Hamilton group, and between the Carboni- 

 ferous Limestone and the Coal Measures." 



*^As remarked above, the Drift deposits, in Michigan, immediately 

 overlie the Carboniferous Formation. The rocks beneath the Drift 

 shew the usual glacial striae, and the lower Drift beds consist of 



