There is some difference of opinion in respect to the extent of this subsidence, and, indeed, respecting the height attained by the Philadelphia brick-clay, or McGee's Columbia deposit. Professor Lewis (whose residence was at Philadelphia, and who had devoted much time to field observations) insisted that the deposit could not be found higher than from 180 to 200 feet above the immediate flood-plain of the river valleys where they occur. But, without entering upon this disputed question, it is sufficient to consider the bearing of the facts that are accepted by all--namely, that towards the close of the Glacial period there was a marked subsidence of the land on the eastern coast of North America, increasing towards the north. Fully to comprehend the situation, we need to bring before the mind some of the indirect effects of the Glacial period in this region. The most important of these was the necessary projection of subglacial conditions over a considerable belt of territory to the south of that actually reached by glacial ice; so that, while there are no clear indications of the existence of local glaciers in the Appalachian Mountains south of the central part of Pennsylvania, there are many indications of increased snow-fall upon the mountains, connected with prolonged winters and with a great increase of spring floods and ice-gorges upon the annual breaking up of winter. These facts have been stated in detail by Mr. McGee,[CV] from whose report it appears that, on the Potomac at Washington, the surface of the Columbia deposit is 150 feet above tide, and that the deposit itself contains many boulders, some of which are as much as two or three feet in diameter. These are mingled with the gravel in such a way as to show that they must have been brought down by floating ice from the head-waters of the Potomac when the winters were much more severe than now. That this deposit is properly the work of the river is shown by the entire absence of marine shells. [Footnote CV: Seventh Annual Report of the United States Geological Survey for 1885 and 1886, pp. 537-646.]