Page:Popular Science Monthly Volume 30.djvu/18

8 of the Tertiary ages, except on mountains higher than any the continent now bears. In the Tertiary the climate was sub-tropical over all the area of the United States south of the British line, as is shown by the fact that palms and cinnamon-trees grew as far north as Vancouver's Island and the falls of the Missouri.

The relations which the great Quaternary lakes, Bonneville, La Hontan, etc., bore to the former glaciation of the adjacent mountains is an interesting subject of inquiry. As I have mentioned, it has been suggested that it is the relation of cause and effect, but this is supported by no proof, and opposed by strong circumstantial evidence. The lakes and the glaciers may have been synchronous, and, to some extent, co-operative phenomena; but the relationship was rather fraternal than filial, as they had probably a common parentage.

The cause of the former wide spread of water-surfaces in the undrained portions of the Great Basin was either more copious precipitation or less rapid evaporation than at present. It is well known that the supply of moisture of this region is derived from the rain-bearing winds which blow steadily on to the land from the Pacific, and "the testimony of the rocks" is conclusive to the effect that there has been no change in the outline or elevation of the land, or the relations of land to sea since the Tertiary age, which could have materially increased or diminished the precipitation.

So in regard to the topography of the interior. Since the end of the Tertiary it has remained essentially the same. The hydrographical basins have been filled and emptied, but the old beach-lines which mark their sides prove that the country has remained substantially undisturbed. It is apparent, therefore, that the causes of any variation in the amount of precipitated or accumulated moisture must be climatic and not topographical. King, Gilbert, and Russell have shown that there have been several alternations of wet and dry climate in the Great Basin, and they are substantially in agreement that there have been two wet and two dry periods, of which the last is the present.

It would seem easy to determine by observation the relationship between the lakes and glaciers of that region, since some of the glaciers descended far below the highest water-level, as was the case with the Little Cottonwood glacier, to which reference has already been made, but the actual contact of the glaciated surface and the lake sediments is there covered and concealed by modern débris. The observations made elsewhere by Gilbert and Russell will, when published, probably demonstrate that which can now only be conjectured. "We can confidently predict, however, that it will be found that the same climatic condition which produced the accumulation of water in the lake-basins also caused the accumulation of congealed water on the highlands. A greatly increased rainfall might produce lakes without forming glaciers, but we appeal in vain to the facts or the imagination for a probable cause of an increased oceanic evaporation, with a more abundant