34 REPORT OF THE SECRETARY. 



the portion of sand increases, and also the size and number of the 

 boulders, which are mostty of igneous origin. 



The second member of the glacier drift is not as readily made out 

 as the other two. In general it is thin, passing into the first member 

 above, and into the third below. Its characteristics are the finer 

 condition of the material and more distinct stratification in the alter- 

 nate layers of clay and sand. It is seen in its greatest development 

 at the Grand Sable of Lake Superior, east of Grand island, where the 

 coarse sand forms a stratum of from three to four hundred feet in 

 thickness, overlying a thin stratum of clay. The prominent moun- 

 tains and dunes along the eastern shore of Lake Michigan belong to 

 this member of the drift formation. 



The third member consists of the ash-colored, the red, and the blue 

 laminated clays, the difference of color being probably caused by 

 varying portions of oxide of iron. These strata are not, strictly 

 speaking, formed of clay, but of finely comminuted sand, marl, and 

 oxide of iron, with alumina enough to cause adhesion. On the sum- 

 mit of the land whence the streams flow northwardly into Hudson's 

 bay, southerly into the gulf of Mexico, and southeasterly into Lake 

 Superior, the three members of the drift period are conspicuously ex- 

 hibited — the third occupying the lower level. On Lake Superior the 

 lower member attains its greatest thickness. The rocks on which the 

 glacial drift rests, wherever uncovered, exhibit markings and indica- 

 tions of scouring and striaa with variable distinctness, depending upon 

 the capability of the rock to retain the impressions. In some cases 

 the movement was parallel, or nearly so, to the strike of the strata, 

 and in such instances sandstone beds have been carried away to a 

 considerable depth, leaving long, narrow ridges of material better 

 able to resist the grinding action. The general movement, as indi- 

 cated by the striae, was from the northeast to the southwest, the same 

 as that observed in the northern part of Europe. In New England, 

 markings of this kind on the rocks have been observed at 3,000 feet 

 elevation, in Ohio at from 1,300 to 1,400 feet, and at Point Keewenaw, 

 in Lake Superior, at from 1,400 to 1,600 feet. These elevations indi- 

 cate the great thickness which the glacial ice must have attained. 

 In various parts of the drift, boulders and nuggets of copper occur, 

 which, in some cases, have been transported a long distance south- 

 ward. Those found near Lake Superior, which have not been carried 

 far from their origin, are of great size, and not as much rounded by 

 attrition as those found at a greater distance. One of the former, 

 from the clay on the west fork of the Ontonagon river, now in the 



