STATE GEOLOGIST. 63 



the formation seen at Thunder Bay Island. The oolitic portion 

 seen at the summit of 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 Raisinville, 8 miles from 

 Monroe. The whole thickness of the formation in Monroe 

 county cannot be over 50 or 60 feet from the oolitic beds to the 

 Onondaga Salt Group, while at Mackinac the same strata 

 attain a thickness of 215 feet. • 



To the west of Mackinac, the Helderberg limestones are 

 found underlying the numerous islands near the foot of Lake 

 Michigan, and forming the highlands seen a few miles back 

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

 At the head of this bay, they are seen forming cliffs along the 

 shore. The highest beds, are thick, light, argillo-calcareous, reg- 

 ularly stratified, abounding in Brachiopods, geodes and long 

 cylindrical cavities. At some points these beds are made up 

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

 Bay Island. A calcareo-argillaceous, shaly layer, of a dark 

 gray color, ,one or two feet thick, separates these upper beds 

 from a pale buff, argillo-calcareous, thick bedded, fissile mass, 

 4 feet thick, which is underlain by 3| feet of a light dingy gray 

 argillo-calcareous, porous, geodiferous mass, breaking with a 

 very uneven fracture. Still lower we find 6. feet of light argil- 

 laceous, fine grained limestone, resembling that of the Clinton 

 Group. We next come to a light buff limestone, much shat- 

 tered, destitute of fossils, 6 feet thick, apparently representing 

 the brecciated mass about Mackinac. Finally, at the lowest 

 points, is seen a light buff limestome, banded with argillacaous 

 matter, and resembling the highest beds of the Onondaga Salt 

 Group. 



The Helderberg limestones of Michigan are well stocked 

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

 tered with the drift to all parts of the State Probably three- 

 fifths of all the fossils picked up from the surface of the Lower 

 Peninsula — except in the iinmediate vicinity of the outcrop of 



