NORTH AMERICA IN THE ICE PERIOD. 229 



The map of Lake Tahoe was begun in the summer of 1876, in 

 which season a survey of the shores of the lake and the mountains im- 

 mediately outlying was made by Lieutenant M. M. Macomb, of the 

 army, and the writer. Dr. F. Kampf, Mr. Gilbert Thompson, Lieuten- 

 ant S. E. Tillman, Mr. William A. Cowles, Mr. Frank O. Maxson, Mr. 

 Anton Karl, and other engineers have contributed to the work at dif- 

 ferent times. In one respect it is still incomplete, and it is to be 

 hoped that provision may yet be made for a series of soundings with 

 which to illustrate the very remarkable configuration of the bottom of 

 the lake. It is scarcely to be doubted that the geographical map of 

 the future, and especially the geological chart, will portray surfaces 

 below as well as above water. These could be drawn in contour-lines, 

 either faint, broken, or in blue color, so as to indicate their submarine 

 position. The " ripple-lines " with which shores are now sometimes 

 represented are arranged at regular intervals, in curves which are 

 regularly concentric, and they give no idea of the additional character 

 which a map would derive from a real plot of its surface below water. 



Since the interest and effect of a plot increase with the irregular- 

 ity of the surface of which it is a copy, no known body of water excels 

 Lake Tahoe in the instruction to be gained from such a survey. It is 

 very deep in comparison with its narrow superficial extent, the few 

 random soundings already taken showing a depth of over sixteen hun- 

 dred feet, with indications that there are other crater-like depressions 

 beyond any yet discovered. The absence of currents and other sub- 

 surface agencies of disturbance has probably left the cliffs and canons 

 beneath the lake as rugged and unworn as those above it. The western 

 bank is apparently abrupt and even precipitous, while the eastern 

 shore is more gentle in its descent. Thus these declivities correspond 

 to the two mountain-slopes of which they are respectively prolonga- 

 tions. A hydrographic survey would trace them in their descent, 

 fathom the unknown depths to which they go, and furnish informa- 

 tion which would not only be of general scientific and perhaps eco- 

 nomical interest, but would also reveal the geological secret of the 

 structure of this wonderful basin in the mountain-top. 



NOETH AMEEICA IN THE ICE PEEIOD. 



By C. H. HITCHCOCK, 



PROFESSOR OF GEOLOGY IX DARTMOUTH COLLEGE. 



FEW geological subjects have been discussed so much as the nat- 

 ure, extent, and cause of the glacial period. At first, the specu- 

 lations of such men as Dr. Buckland upon the ice-markings excited de- 

 rision, and led to the publication of caricatures. Next, when its claims 



