STATE GEOLOGIST. 95 



atmosphere and the lake, without undergoing important changes 

 from its original character. Especially are we unable to decide 

 from this exposure, as to the whole probable thickness of the 

 gypseous deposites. The waves of Lake Huron have for ages 

 been breaking against the exposed edges of the strata, and the 

 gypsum has necessarily been dissolved out to a considerable 

 distance back from the shore. In confirmation of this inference 

 I found at Plaster Point, one mile north of the north line of Bay 

 county, numerous " sink holes," as if produced by the subsi- 

 dence of the overlying beds, after the dissolving out of the 

 gypsum. Some of these are eight feet deep. Water is stand- 

 ing in them, probably at the level of the lake. The sides are 

 steep, exactly as if the rocks had sunken. In one place a sink 

 is seen pursuing an irregular course for several reds, toward 

 the lake, and the whole appearance is exactly such as is pro- 

 duced by the falling in of the roof of a miner's " drift." Off 

 this point, in calm water, the bed of the lake is seen to be a 

 mass of pure white gypsum — the same, undoubtedly, which 

 rises above the surface at Plaster Bluff half-a mile further north. 



The land, back from the shore of Tawas Bay, rises in a suc- 

 cession of ridges running parallel with the lake. About 12 

 miles back, in the vicinity of the Au Gris river, the country 

 becomes very broken, resembling that upon Grand River in the 

 vicinity of the gypsum quarries. At the residence of Sherman 

 Wheeler, i\ miles south of Tawas City, one of the parallel 

 ridges has attained an elevation of 40 feet, and the acclivity 

 facing the lake presents a slope of 30° with the horizon. This 

 ridge is said to increase in height as far north as the Tawas 

 River. Mr. Wheeler informed me that no explorations had 

 ever been made in this bluff, except to a limited extent by Mr 

 Challis. Strata were found, called clay by Mr. Wheeler, though 

 the specimen shown me was the brown limestone ef the Michi- 

 gan Salt Group, as seen at Plaster Bluff. The green streaks in 

 it were pronounced by Mr. Challis to be indications of the 

 proximity of copper. Coal was also prophesied in this ridge. 



Arriving at Tawas City, I obtained some further information 



