HELDERBERG GROUP. 35 



An analysis of the dolomite with acicular crystals gave, 



Carbonate of lime 59 per cent. 



" " magnesia 39 " 



Insoluble residue not weighed. 



The acicular spar crystals are very characteristic of the lime- 

 stones of this lower horizon of the Helderberg group, everywhere 

 to be found in this district, in Ohio, in Goderich, Canada, and in 

 Mackinac Island where these beds are uncovered. Similar aci- 

 cular limestones are found in lower geological position in the dolo- 

 mite beds of the Onondaga group, and in certain beds of the 

 Niagara group. (See report on the Upper Peninsula, page 35.) 



Prof. A. Winchell identified the dolomites of the Ida quarries 

 with the Onondaga group, but the position of these beds is not 

 lower than that of the other quarries considered to represent the 

 water-lime group, and no difference in the lithological characters 

 of the strata exists which would justify a distinction. In the 

 deep boring made in the Court-house yard of Monroe for a depth 

 of 300 feet, no particular change in the nature of the rock was ob- 

 served ; nearly all this thickness was made up by dolomites, some 

 of which, by their cellulose character, with cavities once filled with 

 crystals, are similar to beds found in the Onondaga group, but no 

 gypsum beds nor shale deposits, which are significant of this group, 

 were noticed. 



The gypsiferous rock beds have been found in the State of 

 Ohio at Sylvania, but in Michigan no positive evidences of the 

 development of this formation have been discovered in the south- 

 eastern end of the State. Remarkable in this region are the 

 numerous sink-holes found over it. Some of them are small, 

 abrupt, funnel-shaped depressions ; others are larger and sink more 

 gradually. Many of them are filled with water and form respect- 

 able lakes, as Ottawa Lake and Little Lake, while others are 

 dry or filled only for a part of the year. The water in all these 

 lakes is subject to considerable fluctuations during the different sea- 

 sons. Ottawa Lake, which covers about a square mile of surface, 

 has in the spring of the year a depth of 50 feet ; toward the fall 

 it has lowered its level about 25 feet below high-water mark, 

 and its shallowest parts lie perfectly dry. The water usually dis- 

 appears rapidly when it begins to sink, which is not the effect of 



