Page:Popular Science Monthly Volume 9.djvu/303

Rh and investigation, to determine if possible the causes of these remarkable changes of climate.

Up to the present time, however, no theory has been proposed which has been sustained by really satisfying evidence, and there is still much difference of opinion on the question among those who know most about it.

As the subject is one of peculiar geological significance, and great dramatic interest, I venture to bring forward some notes upon it, taken from the geologist's standpoint, hoping that they may contribute in some slight degree to the solution of the problem.

The theories which have been proposed to account for the cold of the Ice period divide themselves into two groups, viz., the cosmical and terrestrial; or those which invoke extraneous or astronomical influences, and those which look to changes in the earth itself, or on its surface, for a sufficient cause or causes.

In the first category may be enumerated the theory of Prof. Croll, that variations in the eccentricity of the earth's orbit have induced great alternations of climate on portions of the earth's surface; that of Belt and Drayson, which supposes the known variability of the angle of the pole with the ecliptic to have been at times sufficiently great to have brought arctic conditions locally down into the temperate zone; also, the speculations that the heat evolved from the sun has been variable in quantity, that the earth has at various times passed through cold spaces in the universe, etc.

In the second category are the views first put forth by Lyell, according to which all the variations of climate recorded in geological history have been induced by changes in the earth itself or on its surface.

In this paper I shall consider only the latter theory, leaving the discussion of the astronomical aspects of the subject to astronomers, mathematicians, and physicists, who alone are competent to thoroughly investigate them.

The explanation given by Lyell of the cold of the Ice period is in conformity with his characteristic conservatism. It is well known that the climatic conditions of all parts of the earth's surface are profoundly affected by their topographical features. This may be seen at a glance by reference to any map on which the isothermal lines are delineated. Continental surfaces are known to be productive of extremes of temperature, while the climate of sea areas is comparatively equable; and the general character of the climate of land and water surfaces is further and locally affected by the configuration and altitude of the land, by the breadth and depth of the oceanic basins, and especially by the ocean-currents. The sea forms the great evaporating surface, and the source from which is derived the enormous quantity of water transported by the system of atmospheric circulation. The local climate of continents is also largely influenced by the winds