I. THE PRE-CAMBRIAN HISTORY OF THE QUARTZITE.

From loose sand to quartzite.—To understand the geography of a region it is necessary to understand the nature of the materials, the sculpture of which has made the geography.

It has already been indicated (p. [14]) that the Huronian quartzite of which the most prominent elevations of this region are composed, was once loose sand. Even at the risk of repetition, the steps in its history are here recounted. The source of the sand was probably the still older rocks of the land in the northern part of Wisconsin. Brought down to the sea by rivers, or washed from the shores of the land by waves, the sand was deposited in horizontal or nearly horizontal beds at the bottom of the shallow water which then covered central and southern Wisconsin. Later, perhaps while it was still beneath the sea, the sand was converted into sandstone, the change being effected partly by compression which made the mass of sand more compact, but chiefly by the cementation of its constituent grains into a coherent mass. The water contained in the sand while consolidation was in progress, held in solution some slight amount of silica, the same material of which the grains of sand themselves are composed. Little by little this silica in solution was deposited on the surfaces of the sand grains, enlarging them, and at the same time binding them together. Thus the sand became sandstone. Continued deposition of silica between and around the grains finally filled the interstitial spaces, and when this process was completed, the sandstone had been converted into quartzite. While quartzite is a metamorphic sandstone, it is not to be understood that sandstone cannot be metamorphosed in other ways.

Uplift and deformation. Dynamic metamorphism.—After the deposition of the sands which later became the quartzite, the beds were uplifted and deformed, as their present positions and relations show (p. [16]). It is not possible to say how far the process of transformation of sand into quartzite was carried while the formation was still beneath the shallow sea in which it was deposited. The sand may have been changed to sandstone, and the sandstone to quartzite, before the sea bottom was converted into land, while on the other hand, the formation may have been in any stage of change from sand to quartzite, when that event occurred. If the process of change was then incomplete, it may have been continued after the sea retired, by the percolating waters derived from the rainfall of the region.

Either when first converted into land, or at some later time, the beds of rock were folded, and suffered such other changes as attend profound dynamic movements. The conversion of the sandstone into quartzite probably preceded the deformation, since many phenomena indicate that the rock was quartzite and not sandstone when the folding took place. For example, the crushing of the quartzite (now re-cemented into brecciated quartzite) at Ablemans probably dates from the orogenic movements which folded the quartzite, and the fractured bits of rock often have corners and edges so sharp as to show that the rock was thoroughly quartzitic when the crushing took place.

The uplift and deformation of the beds was probably accomplished slowly, but the vertical and highly tilted strata show that the changes were profound (see Fig. [4]).

The dynamic metamorphism which accompanied this profound deformation has already been referred to (p. [15]). The folding of the beds involved the slipping of some on others, and this resulted in the development of quartz schist along the lines of severest movement. Changes effected in the texture and structure of the rock under such conditions constitute dynamic metamorphism. In general, the metamorphic changes effected by dynamic action are much more profound than those brought about in other ways, and most rocks which have been profoundly metamorphosed, were changed in this way. Dynamic action generates heat, but contrary to the popular notion, the heat involved in profound metamorphism is usually secondary, and the dynamic action fundamental.

At the same time that quartz schist was locally developed from the quartzite, crushing probably occurred in other places. This is demorphism, rather than metamorphism.

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.