Page:Popular Science Monthly Volume 49.djvu/179

Rh exclusion of the sea, of which there is no evidence, some have supposed that these waters of the lake district were held at high altitudes by glacial dams for long ages. This hypothesis, notably advocated by Prof. G. K. Gilbert, although based upon negative evidence to explain some difficulties, which are also applicable to southern regions even within the tropics, has retarded the researches into the history of the lakes, and had it been followed would have prevented the discovery of some of the greatest changes in the geography of northeastern America. Indeed, the greater portion of the modern elevation of the region has been

recorded in the tilting of the beach lines recognized by all. But the period of glacial conditions was prior to the lacustrine.