GEOLOGICAL STRUCTURE. I 5 



in all parts of the State. Several localities arc also known where 

 ore deposits of larger extent have been discovered, but up to the 

 present time no practical use has been made of them, and it is 

 questionable whether any of these deposits is of sufficient magni- 

 tude to be an inducement for the erection of furnaces, or for their 

 mining and transportation to other melting works. 



Calcareous tufa, a deposit of springs, analogous to bog iron, is 

 sometimes found in large masses spread over springy hillsides. 

 Locally, this tufa is used for lime-burning; sometimes, however, the 

 porous rock mass is sufficiently compact and hard to be used as a 

 building stone, but I have not often seen it so employed in Michi- 

 gan. 



The bottoms of a number of inland lakes are covered with a 

 peculiar modification of calcareous tufa, a white, marly substance, 

 in part composed of the shells of sweet-water molluscs, mixed with 

 white pulverulent carbonate of lime deposited from solution by the 

 lake water. Pine Lake, near Charlevoix, one of the largest lakes 

 in the north part of the peninsula, has its entire bottom covered 

 by this marl, and to a greater or less extent the same is the case 

 with a great many others. I am not aware that any practical use 

 has been made of these shell marls. They would certainly be good 

 fertilizers. 



As another surface deposit, peat has been mentioned. In- 

 numerable larger and smaller patches cover the swampy surface 

 depressions throughout the whole State. It is formed by a con- 

 tinued growth of a certain class of plants in wet places, where, 

 submerged, the basal parts of the plants die off, and become de- 

 composed and changed into a coaly substance, which accumulates 

 sometimes to very great thickness. Peat is at present little used 

 in Michigan. Several experiments to prepare it for commerce by 

 compressing it with hydraulic machines have given very good 

 results, as far as its usefulness and heating qualities are concerned, 

 but in the light of a pecuniary transaction the results were less 

 favorable. Fuel is too cheap yet, but the time will come when 

 peat will be appreciated at its true value. 



