60 REPORT OF THE 



information furnished by Dr. Miles, is a strong and copious salt 

 ■spring, located upon Harbor Island in the west arm of Grand 

 Traverse Bay. This is now overflowed by the waters ot the lake, 

 but tradition says that the Indians formerly manufactured salt 

 at this place, when the water was several feet lower. It ap- 

 pers quite possible; therefore, that borings which should pene- 

 trate this group of rocks might be rewarded by a profitable 

 supply of brine. 



One other suggestion may be made in connection with the 

 ■economy of this group. The brown and banded argillaceous 

 limestone, which, in Monroe county, generally occurs in the 

 •deeper parts of the quanies, may, on trial, be found to produce 

 a valuable water lime. The trial, if never made, should, by all 

 means, be undertaken. Even should this experiment fail, the 

 hydraulic character imparted to the quicklime manufactured 

 from this rock, or from rock with which this is mixed, must add 

 ■materially to the cementing properties of the lime, provided 

 it is used with reference to the peculiar nature of hydraulic - 

 •cements. 



HI. — DEVONIAN SYSTEM. 



8. — Upper Helderberg Group. 



In the lower part of the cliffs known as Chimney Rock and 

 Lover's Leap, on the west side of Mackinac Island, is seen a 

 •cherty and agatiferous conglomerate, irregularly disposed, but 

 pretty persistent. On the main land west, close to the water's 

 «dge, and beneath the brecciated mass, presently referred to, is 

 found a better characterized conglomerate, a few feet in thick- 

 ness. These beds, occupying the place of the Oriskany Sand- 

 stone of New York, and corresponding N to it in lithological 

 characters, as seen at some of its exposures, may not improb- 

 ably be regarded as representing that formation. The uncer- 

 tainty of the identification, however, prevents me from givino- it 

 -a distinct place in the enumeration of our strata. 



Above this curious conglomerate, rises one of the most 

 jemarkable masses of rock to be seon in this or any Stale. 



