When the extreme hardness of the quartzite is remembered and also the extent of the erosion which affected it (Fig. 4) before the next succeeding formation was deposited, it is safe to conclude that the period of erosion was very long. _Thickness of the quartzite._--The thickness of the quartzite is not known, even approximately. The great thickness in the south range suggested by the diagram (Fig. 4) may perhaps be an exaggeration. Faulting which has not been discovered may have occurred, causing repetition of beds at the surface (Fig. 6), and so an exaggerated appearance of thickness. After all allowances have been made, it is still evident that the thickness of the quartzite is very great. II. THE HISTORY OF THE PALEOZOIC STRATA. _The subsidence._--Following the long period of erosion, the irregular and almost mountainous area of central Wisconsin was depressed sufficiently to submerge large areas which had been land. The subsidence was probably slow, and as the sea advanced from the south, it covered first the valleys and lowlands, and later the lower hills and ridges, while the higher hills and ridges of the quartzite stood as islands in the rising sea. Still later, the highest ridges of the region were themselves probably submerged. [Illustration: Fig. 6.--A diagrammatic cross-section, showing how, by faulting, the apparent thickness of the quartzite would be increased.] _The Potsdam sandstone (and conglomerate)._--So soon as the sea began to overspread the region, its bottom became the site of deposition, and the deposition continued as long as the submergence lasted. It is to the sediments deposited during the earlier part of this submergence that the name _Potsdam_ is given. The sources of the sediments are not far to seek. As the former land was depressed beneath the sea, its surface was doubtless covered with the products of rock decay, consisting of earths, sands, small bits and larger masses of quartzite. These materials, or at least the finer parts, were handled by the waves of the shallow waters, for they were at first shallow, and assorted and re-distributed. Thus the residuary products on the submerged surface, were one source of sediments. From the shores also, so long as land areas remained, the waves derived sediments. These were composed in part of the weathered products of the rock, and in part of the undecomposed rock against which the waves beat, after the loose materials had been worn away. These sediments derived from the shore were shifted, and finally mingled with those derived from the submerged surface. So long as any part of the older land remained above the water, its streams brought sediments to the sea. These also were shifted by the waves and shore currents, and finally deposited with the others on the eroded surface of the quartzite. Thus sediments derived in various ways, but inherently essentially similar, entered into the new formation.