Among the most impressive instances of boulders evidently transported by glacial ice, rather than by icebergs, were some which came to my notice when, in company with the late Professor H. Carvill Lewis, I was tracing the glacial boundary across the State of Pennsylvania. We had reached the elevated plateau (two thousand feet above the sea) which extends westwards and southwards from the peak of Pocono Mountain, in Monroe County. This plateau consists of level strata of sandstone, the southern part of which is characterised by a thin sandy soil, such as is naturally formed by the disintegration of the underlying rock, and there is no foreign material to be found in it. But, on going northwards to the boundary of Tobyhanna township, we at once struck a large line of accumulations, stretching from east to west, and rising to a height of seventy or eighty feet. This was chiefly an accumulation of transported boulders, resembling in its structure the terminal moraines which are found at the front of glaciers in the Alps and in Alaska, and indeed wherever active glaciers still remain. But here we were upon the summit of the mountain, where there are no higher levels to the north of us, down which the ice could flow. Besides, among these boulders we readily recognised many of granite, which must have come either from the Adirondack Mountains, two hundred miles to the north, or from the Canadian highlands, still farther away. Limiting our observations simply to the boulders, we should indeed have been at liberty to suppose that they had been transported across the valley of the Mohawk or of the Great Lakes by floating ice during a period of submergence. But we were forbidden to resort to this hypothesis by the abrupt marginal line, running east and west, upon Pocono plateau, along which these northern boulders ceased. South of this evident terminal moraine there was no barrier, and there were no northern boulders. On the theory of submergence, there was no reason for the boundary-line so clearly manifested. Ice which had floated so far would have floated farther. Still further, on going a few miles east of the Pocono plateau, one descends into a parallel valley, lying between Pocono Mountain and Blue Mountain, and one thousand feet below their level. But our marginal southern boundary of transported granite rocks did not extend much farther south in the valley than it did on the plateau, except where we could trace the action of a running stream, evidently corresponding to the subglacial rivers which pour forth from the front of every extensive glacier. In these facts, therefore, we had a crucial test of the glacial hypothesis, and, in view of them, could maintain, against all objectors, the theory of the distant glacial transportation of boulders, even over vast areas of the North American continent.