Page:Popular Science Monthly Volume 35.djvu/581

Rh, species from far north having been driven southward by the severe climate and accumulating ice, as is sown by remnants of a flora and fauna like those of the arctic regions, which have managed to continue their existence since the Ice age on the tops of mountains in temperate latitudes. Many peculiarities in the distribution of forest trees, made known by the researches of the late Prof. Asa Gray, also find their only adequate explanation in these vicissitudes of climate.

Northwestern Europe was covered by an ice-sheet about half as extensive as that of our own continent, and the author gives on a single map a comparative view of the glaciated areas of both. Another map shows the course of the terminal moraines recently traced by Lewis in Ireland, Wales, and England, and by Salisbury in Germany, each of whom had much previous experience from work on glacial geology in the United States.

Treating of the cause and date of the Glacial period, Prof. Wright rejects the astronomic theory of Croll and Geikie, which attributes the severe climate to conditions dependent on the eccentricity of the earth's orbit between two hundred and forty thousand and eighty thousand years ago. Instead of this, the post-glacial erosion of the gorge below the Falls of Niagara and of that extending eight miles on the Mississippi from Fort Snelling to the Falls of St. Anthony at Minneapolis, similar erosion by streams tributary to Lake Erie, changes in the shores and deposits of dune sand about Lake Michigan, and other observations, afford much shorter measures of the time since the departure of the ice-sheet, agreeing in their testimony that it was no longer ago than seven to ten thousand years. Prof. Wright is also disposed to doubt that there have been two distinct Glacial epochs in America, and believes that the facts thus far obtained are capable of explanation on the theory of but one epoch, with the natural oscillations accompanying the retreat of so vast an ice front.

The last two chapters review the evidences of man's presence in America and Europe during the Glacial period, specially describing the important discoveries of paleolithic implements in glacial gravel deposits near Trenton, N. J., by Abbott; near Claymont, Del., by Cresson; in the Little Miami Valley, Ohio, by Metz; and at Little Falls, Minn., by Miss Babbitt. But doubts remain concerning the authenticity of the famous Calaveras skull and stone implements denoting a higher state of development than that of palæolithic man, reported as occurring in the lava-covered gold-bearing gravels of California, which, if obtained there in the undisturbed gravel, would give to our race a considerably greater antiquity than is otherwise known.

In the appendix Mr. Upham contributes "an explanation of the causes of the Glacial period, which, in this application of its fundamental principle, seems to be new, while in its secondary elements it combines many of the features of the explanations proposed by Lyell and Dana and by Croll. Briefly stated, the condition and relation of the earth's crust and interior appear to be such that they produce, in connection with contraction of the earth's mass, depressions and uplifts of extensive areas, some of which have been raised to heights where their precipitation of moisture throughout the year was almost wholly snow, gradually forming thick ice-sheets; but under the heavy load of ice subsidence ensued, with correlative uplift of other portions of the earth's crust; so that glacial conditions may have prevailed alternately in the northern and southern hemispheres, or in North America and Europe, and may have been repeated after warm interglacial epochs." Mr. Upham believes that the earth's crust floats in a condition of hydrostatic equilibrium upon the heavier liquid or viscous mobile interior, or layer enveloping the interior, subject, however, to strains and resulting deformation because of the earth's contraction. But such oscillations seem not inconsistent with the doctrine that the earth's interior is solid, with a degree of mobility like that of ice in glaciers. Whether the formation of the Himalayan mountain range has been contemporaneous and correlative with the Glacial period, and the Appalachian uplift with the Carboniferous and Permian glaciation of portions of the Eastern hemisphere, as is here suggested, must probably require many future years of observation and study to determine.

All who have read the earlier work of Prof. Geikie, or listened to Prof. Wright's