STATE GEOLOGIST. 129' 



Upon the smooth and striated surfaces of the rocks, has been 

 brought an immense deposite of waterworn and comminuted 

 materials, derived from the breaking up and disintegration of 

 pre-existing strata. We generally — almost universally — find 

 the face of the rock overspread with a confused mixture of blue 

 clay and azoic and plutonic boulders and pebbles. These coarse 

 materials are often arranged in rude courses -which have a 

 curved or irregular dip, and may often be seen outcropping on 

 a hill : side, or even upon the plain. At East Saginaw these ma" 

 terials are 90 to 100 feet below the surface. At Detroit they 

 lie 130 feet below the siirface. Through the interior of the 

 State they are found outcrepping at irregular intervals, produc- 

 ing occasional patches of ground principally noteworthy for 

 their cobble stones. A field was noticed in the southern part 

 of Jackson county in which, by measurement, the average dis- 

 tance between adjacent stones was only four inches. This 

 small field had already furnished many hundred cords of these 

 stones; but every plowing seemed to favor the development of 

 a new crop. Strange to say, this and similar lands are found 

 to produce excellent crops of wheat. 



Great use is made of these cobble stones for purposes of pav- 

 ing in the cities, a use for which their great hardness and 

 toughness renders them eminently fit. Mineralogically, they 

 consist mostly of rounded fragments of syenite, greenstone, 

 vitreous and jaspery sandstones, and hornblendic, talcose, and 

 setpentinous rocks of the azoic series. 



Above the boulder bed we find a deposite of argillaceous 

 and arenaceous materials more distinctly stratified and assorted, 

 as if by the action of eddying waters. So far as I have ob- 

 served, the lake ridges and terraces are worked in these mate- 

 rials. Here we find buried, numerous tree trunks, generally of 

 the White Cedar, many of which may be seen projecting from 

 the bank which overhangs Lake Huron, near Fort Gratiot, and 

 at numerous other points onthe lakes. 



The materials of this assorted drift are not so exclusively of 

 extreme northern origin as those of the boulder drift. Perhaps 

 It 



