4 [358] A. Winchell on the Rocks of Michigan. 
the Marshall sandstone near the center of Ottawa county. To¬ 
ward the northeast, the next actual exposure occurs near the 
sources of the Cass river in Sanilac county, whence the sand¬ 
stones are traceable to the shore of Lake Huron, which they 
occupy from the mouth of Pigeon river to Pt. aux Barques. 
The fine-grained sandstones of the southern part of the state 
are overlaid by the saliferous and gypsiferous clays of the 
“Michigan Salt Group,” attaining a thickness of 184 feet at 
Grand Rapids and Saginaw, but in Jackson county attenuated 
to less than 50 feet. They are underlaid in Branch, Calhoun, 
St Joseph and Yan Buren counties, by a considerable thickness 
of argillaceous strata sometimes plastic and abounding in Kid¬ 
ney iron ore, sometimes shaly, and sometimes blackened with 
bituminous matter. They contain very few fossils. 
In the vicinity of Pt. aux Barques, the typical Marshall sand¬ 
stone is separated by two feet of conglomerate from the bluish, 
fine-grained, homogeneous gritstone, from which have been 
manufactured the famous Huron grindstones. These are suc¬ 
ceeded downwards by a great thickness of dark, fissile, some¬ 
what bituminous shales, with intercalated flagstones, striking 
nearly southward in lines parallel with the lake shore in Huron 
and Sanilac counties, and occupying, as is believed, a large por¬ 
tion of the bed of the lake between Saginaw Bay and Port 
Huron. At a lower level occur the black bituminous shales of 
Kettle Pt. on the Canadian shore, and Sulphur island and Squaw 
Pt. at the head of Thunder Bay on the Michigan shore. At 
the latter localities, these are succeeded by the fossiliferous lime¬ 
stones of the Hamilton group. The black shales are seen again 
near the mouth of Grand Traverse Bay, covered by a few feet 
of green shale, and underlaid as before by the Hamilton lime¬ 
stones. No rocks of the age of the Hamilton Group have been 
recognized as yet, in the southern part of the state. 
The following is a synoptical view of the strata embraced in 
the foregoing sketch; aggregate thickness over 800 feet. 
Carboniferous limestone, ..... 66 feet. 
Michigan Salt Group, ..... 184 “ 
Napoleon Group, ..... 123 “ 
(d) Shaly micaceous sandstone, . 15 “ 
(c) Napoleon sandstone,—highly saliferous in many 
localities,. 78 u 
(6) Shaly, micaceous sandstone,. 15 “ 
(а) Clay and shale, more than . 15 “ 
Marshall Group, .. 173 “ 
(d) Reddish, yellowish and greenish sandstones,.. 147 “ 
(c) Shaly, micaceous sandstone, .. 10 “ 
(б) Conglomerate, . 2 “ 
(a) Fine, bluish gritstones, .. 14 “ 
