STATE GEOLOGIST. . 65 



fer gregaria, Merista, Gyrtia, (n. sp.) Acervularia Davidsoni 

 (from the limestones separating the Belderberg and Hamilton 

 Groups,) and numerous other fossils. Prom other parts of the 

 State, this group has furnished a Bellerophon, (n. sp.) Spirifer 

 ocuminatus, (cultrijugatus,) Syringopora, Chonetes, Productust 

 Atrypa reticularis, Strophomena ruqosa, Spirifer duodenaria, 

 Strophomena hemispherica, Atrypa (n. sp.), Spirifer (peculiar for 

 plication in mesial sinus) Strophodonta (n. sp.) Meristella, ( N. 

 Y. Kegents Rep. 1859,) and many other forms. 



The formation is extensively intersected by divisional planes; 

 and even in those portions not belonging to the brecciated mass 

 at Mackinac, is apt to be considerably broken up. The open 

 character of the rock permits the escape of numerous copious 

 springs of fresh water, and occasionally gives rise to the sud- 

 den disappearance of streams and lakes. Various accounts are 

 current, in Ttfonroe county, of subterranean communications 

 from lake to lake, and even between Lake Erie and the western 

 part of the county. I heard it repeatedly stated, that at cer- 

 tain seasons of the year, Ottawa Lake passes off by some 

 subterranean outlet, causing the death of all the fish which 

 remain, but that, on the refilling of the lake, the water is always 

 accompanied by a fresh stock of fish. In Mr. James Cummins' 

 quarry, about five miles, in a right line, north-east of Ottawa 

 Lake, the rock is described as cavernous and full of sink holes; 

 and what is remarkable, is the fact that this quarry is always 

 filled with water when the lake is high,' and empty when it is 

 low; and whenever the quarry is full, it contains bass and dog- 

 fish of the common species of that region. 



The curious, suture-like structure so often referred to by 

 other geologists, is frequently met with in Michigan — two con- 

 secutive layers of rock being studded, on their contact surfaces 

 with tooth-like or prism-like processes which fit into correspond- 

 ing pits on the opposite surface. A thin film of black bituminous 

 matter generally prevents a perfect contact of the contiguous 

 surfaces. Sometimes these processes are so little developed, 



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