The ice that buried upper North America acted in the same manner as the Greenland ice to-day: it eroded the mountains, it sent off bergs, and the rocks and gravel that it tore from the hills by a pressure of a thousand tons to the square yard were dropped at its foot, where they formed a moraine, as it is called. These moraines, which may be seen at the feet of the glaciers in Switzerland and British Columbia, and that sometimes make heaps and hills of rock, like rude forts, forty and fifty feet high, are trifling affairs to the shoals left by the great glacier of the ice age, for that can be traced from the Atlantic coast nearly to the Mississippi River. Long Island, measuring approximately 120 miles in length, is a small part of the dump of this glacier, and it is sometimes possible to tell where the stones came from that are found on the surface. For example, there are in Brooklyn anthophyllite from Westchester county, feldspar and green mica from Fort George, basalt from the Palisades of the Hudson, and a block of labradorite was found on Myrtle Avenue that had been carried down from the Adirondacks, three hundred miles. The members of the United States geological survey, supported by the New York and other state surveys, have studied into the course and volume of the glacier and mapped its moraine from Montauk Point westward nearly half across the continent. By this survey we learn that the gneiss that crosses under the East River and approaches the surface at Astoria, is the only bed rock to be found on Long Island, Brooklyn resting on a cushion of glacial drift that in some places is three hundred feet deep. Originally there were cliffs of gneiss edging the Atlantic, but the great glacier shaved these down to mere ledges. Central Park, New York, preserves a number of these ledges, rounded off into "sheep backs" and scratched by the pieces of stone that formed a grinding surface to the under side of the ice, while every now and then a boulder comes to the top of the ground in Brooklyn that is scored and almost polished by rubbing against those ledges. Pieces from that very outcrop in Hell Gate are found in Brooklyn streets. We are also reminded in Mr. Skinner's review that manufacturers of brick, tile, terra cotta, pottery, and porcelain in other states have to rely in part on the clay beds that environ Brooklyn for their material, and, in fact, that clay and sand are the only economic mineral products of Long Island. The explanation of this is that Brooklyn clays are rich in silica, which is apt to be deficient in the clays of New Jersey. Without silica the clays are weak, and bricks and utensils made from them readily crack and crumble; but by mixing properly the best results are obtained. Excellent sand for glass-making is also found in and near Brooklyn.