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Page:Popular Science Monthly Volume 20.djvu/241

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NORTH AMERICA IN THE ICE PERIOD.

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 immediately outlying was made by Lieutenant M. M. Macomb, of the army, and the writer. Dr. F. Kampf, Mr. Gilbert Thompson, Lieutenant S. E. Tillman, Mr. William A. Cowles, Mr. Frank O. Maxson, Mr. Anton Karl, and other engineers have contributed to the work at different 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 irregularity 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 hundred 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 prolongations. A hydrographic survey would trace them in their descent, fathom the unknown depths to which they go, and furnish information which would not only be of general scientific and perhaps economical interest, but would also reveal the geological secret of the structure of this wonderful basin in the mountain-top.

 

NORTH AMERICA IN THE ICE PERIOD.
By C. H. HITCHCOCK,

PROFESSOR OF GEOLOGY IN DARTMOUTH COLLEGE.

FEW geological subjects have been discussed so much as the nature, extent, and cause of the glacial period. At first, the speculations of such men as Dr. Buckland upon the ice-markings excited derision, and led to the publication of caricatures. Next, when its claims