_Erosion of the quartzite._--When the Huronian beds were raised to the estate of land, the processes of erosion immediately began to work on them. The heat and the cold, the plants and the animals, the winds, and especially the rain and the water which came from the melting of the snow, produced their appropriate effects. Under the influence of these agencies the surface of the rock was loosened by weathering, valleys were cut in it by running water, and wear and degradation went on at all points. The antagonistic processes of uplift and degradation went on for unnumbered centuries, long enough for even the slow processes involved to effect stupendous results. Degradation was continuous after the region became land, though uplift may not have been. On the whole, elevation exceeded degradation, for some parts of the quartzite finally came to stand high above the level of the sea,--the level to which all degradation tends. Fig. 4 conveys some notion of the amount of rock which was removed from the quartzite folds about Baraboo during this long period of erosion. The south range would seem to represent the stub of one side of a great anticlinal fold, a large part of which (represented by the dotted lines) was carried away, while the north range may be the core of another fold, now exposed by erosion. Some idea of the geography of the quartzite at the close of this period of erosion may be gained by imagining the work of later times undone. The younger beds covering the quartzite of the plains have a thickness varying from zero to several hundred feet, and effectually mask the irregularities of the surface of the subjacent quartzite. Could they be removed, the topography of the quartzite would be disclosed, and found to have much greater relief than the present surface; that is, the vertical distance between the crest of the quartzite ridge, and the surface of the quartzite under the surrounding lowlands, would be greater than that between the same crest and the surface of the sandstone. But even this does not give the full measure of the relief of the quartzite at the close of the long period of erosion which followed its uplift, for allowance must be made for the amount of erosion which the crests of the quartzite ranges have suffered since that time. The present surface therefore does not give an adequate conception of the irregularity of the surface at the close of the period of erosion which followed the uplift and deformation of the quartzite. So high were the crests of the quartzite ranges above their surroundings at that time, that they may well be thought of as mountainous. From this point of view, the quartzite ranges of today are the partially buried mountains of the pre-Potsdam land of south central Wisconsin.