According to Mr. McGee, also, there is a gradual decrease in the height of these delta terraces of the Columbia period as they recede from the glacial boundary--that at the mouth of the Susquehanna being 245 feet, that of the Potomac 140 feet, that on the Rappahannock 125, that on the James 100, and that on the Roanoke 75; while the size of the transported boulders along the streams also gradually diminishes in the same order. During the Columbia period the Susquehanna River transported boulders fifty times the size now transported, while the Potomac transported them only up to twenty times, the Rappahannock only ten times, the James only five, and the Roanoke only two or three times the size of those now transported. This progressive diminution, both in the extent of the deposit and in the coarseness of the material deposited by these rivers at about the time of the maximum portion of the Glacial period, is what would naturally be expected under the conditions supposed to exist in connection with the great Ice age, and is an important confirmation of the glacial theory. That the period of subsidence and more intense glacial conditions during which the Columbia deposits took place, preceded, by a long interval, the deposition of the gravel terraces at Trenton, N. J., and the analogous deposits in the Mississippi Valley where palæolithic implements have been found, is evident enough. The Trenton gravel was deposited in a recess in the Columbia deposit which had been previously worn out by the stream. Indeed, in every place where opportunity offers for direct observation the Trenton gravel is seen to be distinctly subsequent to the other. It was not _buried by_ the Philadelphia red gravel and brick-clay, but to a limited degree overlies and _buries_ it. The data for measuring the absolute length of time between these two stages of the Glacial period are very indefinite. Mr. McGee, however, supposes that since the Columbia period a sufficient time has elapsed for the falls of the Susquehanna to recede more than twenty miles and for those of the Potomac eighteen miles, and this through a rock which is exceedingly obdurate. But, in channels opening, as these do, freely outward, it is difficult to tell in what epochs the erosion has been principally performed, since there are no buried channels, as in the glaciated area, enabling us to determine whether or not much of the eroding work of the river may have been accomplished in preglacial times.