408 KEPOKT OF NATIONAL MUSEUM, 1899. 



O f gypsum associated with salt and referred by Dana to this ssiinc 

 horizon. The rock is mined at Saltville in Washington County from 

 underground pits, and is used mainly for fertilizing. (Specimens Nos. 

 27129, 27153, U.S.N.M.) 



Gypsum deposits of varying thickness and occurring at various 

 depths below the surface are found continuous over thousands of square 

 miles in northern Ohio, but are at present worked only in Ottawa 

 County at a station on the Lake Shore and Michigan Southern Railway 

 which bears the appropriate name of Gypsum (Specimens Nos. 31624, 

 17969, U.S.N.M.). The associated rocks are Lower Helderberg lime- 

 stones and shales and the beds, which vary from 3 to 7 feet in thick- 

 ness, are found at all depths up to 200 or 300 feet. 



The following is a section of the Ottawa County beds as given by 

 Orton: 



Feet. 



Drift clays 12 to 14 



No. 1. Gray rock, carrying land plaster 5 



Blue shale 



No. 2. Bowlder bed carrying gypsum in separate masses embedded in shaly 



limestone 5 



Blue limestone, in thin and even courses 1 



No. 3. Main plaster bed 7 



Gray limestone in courses 1 



No. 4. Lowest plaster bed, variable 3 to 5 



Mixed limestone and plaster, bottom of quarry. 1 



Sections like the above are stated to be capable of yielding 50,000 

 tons of plaster an acre. 



The purest gypsum of the region occurs in No. 2, the bowlder bed, 

 as given above. It consists of calcareous shales through which are 

 scattered concretionary balls of gypsum varying in diameter from 6 

 to 24 inches. This pure variety is used mainly for terra alba; about 

 40 per cent of the total product has in years past been calcined for 

 use as stucco or plaster of paris and 60 per cent for land plaster. 



At Fort Dodge, in Iowa, is a deposit of quite pure, light gray, regu- 

 larly bedded gypsum, resting unconformably upon St. Louis lime- 

 stone and lower coal strata and overlain by drift. It is supposed to 

 cover an area of some 25 square miles. The material was at one time 

 used for building purposes but proved too soft 2 and is now used 

 mainly for land plaster (Specimens Nos. 26804, 63058, 63059, U.S. 

 N.M.). (See Plate 22.) 



There are large deposits of gypsum in Michigan, the most extensive, 

 so far as explored, being near Grand Rapids, Kent County, in the 

 western part of the State (Specimen No. 56397, U.S.N.M.), and at 

 Alabaster Point, in losco County, on the eastern margin of the State. 



1 Geological Survey of Ohio. Eocnomic Geology, VI, 1888, p. 698. 



2 Stones for Building and Decoration, 2d ed., 1897, p. 76. 



