36 REPORT OF THE SECRETARY. 



drew towards its close, the transporting material changed from one in 

 which ice predominated to a modified one of ice and water. 



Eemarks are also made in this paper on the encroachment of the 

 water upon the land, as indicating the slow but constant changes to 

 which the surface of our earth is subjected. The clay bluffs of the 

 lakes are continually worn away by the action of springs, rains, winds, 

 and waves. They are attacked at the base, partially undermined, 

 and finally precipitated into the water with the trees and whatever 

 may be resting upon them. In due time they are spread over the 

 bottom of the lake, forming new strata to be, perhaps, investigated 

 by future geologists. Lake Erie and Michigan, the shores of which 

 are composed of drift clay, are filling up more rapidly than Lakes 

 Huron and Superior, the coasts of which are more rocky. 



Another phenomenon noticed, is that of lines of boulders within 

 and parallel to the shores of shallow lakes, presenting, in some cases, 

 the appearance of a rude wall or fence of large stones. For example, 

 in Mille lake, Minnesota, there are lines of this kind rising five and 

 six feet above the water level, and formed of stones far too heavy to 

 have been moved by the action of the waves. More than fifty years 

 ago a similar appearance was described by President Dwight, of Yale 

 College, as observed at Salisbury, Connecticut. The movement of 

 the stones towards the shore was in this case clearly indicated by the 

 troughs left in the bottom of the lake visible in clear water. The 

 phenomenon is now satisfactorily referred to the expansion of ice in 

 which the boulders are imbedded in winter. This takes place in 

 every direction from the centre of the lake outward, and is exerted 

 in the case of ice of considerable thickness with almost irresistible 

 energy. The stones are carried forward, and on the thawing of the 

 ice are left in their new position. 



The last section of the paper refers to examples of erosions, also 

 referable to the movement of the ice of the glacial period. These 

 are particularly exhibited in the great projection into Lake Superior 

 from the southern shore called Point Keweenaw, in which are scooped 

 out depressions, forming the basins of lakes, inlets, and harbors, ex- 

 hibiting evidence of the direction and intensity of the transforming 

 force, and it is probable that the shore line, and form of the bottom of 

 the great lakes must have been modified by. the action of the same 

 cause. This work is illustrated by a map of north America, a pro- 

 file of the deposits from Lake Brie to the Lake of the Woods, and 

 eleven wood cuts. 



Respectfully submitted, 



JOSEPH HENRY, 

 Secretary Smithsonian Institute. 



