STATE GEOLOGIST. G3 



jecn at Thunder Bay Island. The oolitic portion 

 seen at the summit ^i' Mackinac Island is recognized at several 

 points in Monroe county, while the arenaceous strata of Craw- 

 ford's quarry are repeated in a beautiful white sand, derived 

 from the disintegration of the rock in liaisinville, 8 miles from 

 Monroe. The whole thickness of the formation in Monroe 

 county cannot he over ;")0 or 60 feet from the oolitic beds to the 

 Onondaga Salt Group, while at Mackinac the same strata 

 attain a thickness of *27f) feet 



T the west of Mackinac, the Selderberg limestones are 

 found underlying the numerous islands near the foot of Lake 

 Michigan, and formiog the highlands seen a few miles back 

 from the coast of the Peninsula, as far as Little Traverse Bay. 



At the head of this hay, they are seen forming dills along the 

 The highest beds an- thick, light, argillo-calcareous, reg- 

 ularly stratified, abounding in Brachiopods, geodes and long 

 cylindrical cavities. At some points these beds are made up 

 ol a 'large dome shaped coral, similar to those seen at Thunder 

 Bay [aland. A calcareo-argiUaoeous, shaly layer, of a dark 

 gray color, one <>r two feet thick, separates these upper beds 

 from a pale hull', argillo-calcareous, thick bedded, fissile mass, 

 4 feet thick, which is underlain by :;.' feet of alight dingy gray 

 argillo-calcareous, por< Liferous mass, breaking with a 



very uneven fracture. Still lower we find ('» fret of light argii- 

 .. ained limestone, resembling that of the Clinton 

 Group. We ui e to a light buff Limestone, much shat- 



ils, *') feet thick, apparently representing 

 the brecciated mass ab mt Mackinac. Finally, at the l. 



a light buflflimestome, banded with argillaceous 



., and resembling the highest beds of the Onondaga Salt 

 lp. 



Selderberg limestones of Michigan art; well stocked 

 with fossil remains, which are found not only in place, but 



tared with the drift to all parts oi the State Probably • 

 fifths of all the fossils picked up from the surface of the L >wer 

 Peninsula— except in the immediate vicinity of the out ;i -p of 



