24 LOWER PENINSULA. 



soon disappear under the drift, and from there to Hammond's 

 Bay no more rock exposures are seen along the shore line. 

 On Round Island and on Bois Blanc Island, limestones of a 

 lower position than those of McGulpin's Point come to the 

 surface. The largest portion of theislands is covered by drift and 

 by gravel and sand accumulations, thrown up at a more recent time 

 by the lake. On Round Island the rock escarpments are, on 

 the north side, formed by thick-bedded, light-colored, somewhat 

 porous dolomitic limestones, projecting in a steep bluff of 

 about 50 feet in height. A dense growth of forest trees 

 covers the declivity, hiding the rock ledges from view, when 

 at a distance from the bluff. 



On Bois Blanc the exposures are on the southwestern 

 shore, rising in bluffs to about 15 feet above the water level. 

 Under a cover of boulder drift we find highest, thick ledges 

 of a brittle, dolomitic, and partly cherty limestone ; the lower 

 beds are thinly laminated slabs of a bluish mottled lime rock 

 with rough, earthy fracture ; they are seen extending far out in 

 the bottom of the lake. In the upper thick beds, not unfre- 

 quently the often-mentioned acicular crystals are found pervading 

 the mass, which are characteristic of the lower horizons of the 

 Helderberg group. Fossils have not been found. The lime rock 

 of both islands is used to a small extent for the production of lime, 

 but as a building-stone the rock has little value. 



From the mouth of Oqueok River in Hammond's Bay, onward 

 to Presque Isle, frequent rock exposures can be observed along 

 the shore, which generally is occupied by sand or gravel beaches ; 

 the rocks come out at the ends of projecting land-spurs. The 

 strata seen in these exposures have a higher position than those 

 of McGulpin's Point ; they are the transitory beds between the 

 Helderberg and Hamilton groups, which insensibly merge into 

 each other, without having any sharp line of demarkation. 



I preferred to describe these latter beds in connection with the 

 Hamilton strata, -and have delineated on the geological map ac- 

 companying this report the Helderberg group as confined to the 

 narrow spur of McGulpin's Point and to the islands north and west 

 of it. 



Rocks of the Helderberg group compose the surface of a re- 

 stricted area in the southeast corner of the State, comprising 



