Page:Popular Science Monthly Volume 19.djvu/243

Rh world be sunk and raised, preserving, possibly, through all the changes the rudimentary outlines of hills and valleys by which it was first characterized and marked. And imagining that every new stratum was ingeniously varied in the color and to some extent in the nature of its mineral constituents, and that, upon each reappearance of this diminutive continent, the skillful experimentalist spread a new form of life, what at last might be, after some exposure to weathering and change, the character of its surface? Evidently something like this: in the first place, since, in consequence partly of denudation and partly of only partial submergence, certain tracts will appear cleared of the later deposits, we will find numbers of the whole series laid bare in spots from the highest point, where, as presupposed, only the primitive layer is seen, to the outskirts of the island where the latest layer forms the surface; or to interior depressions, where lake-like centers existed alternately filled and emptied with each recurring deluge. Endless diversity might be introduced into such a scheme, so far as local detail is concerned, but under the conditions given the island would exhibit lines of stratification, each distinguished by color or fossils, and following each' other in the order instituted by the youthful world-makers. Along the crevices and tiny gullies we might detect the minute succession of various strata, and at intervals fragments of the buried life would be revealed. Enlarge this minute illustration till it assumes continental dimensions, reverse the periodic inundations from the water rising to periodic inundations from the land sinking, and in a rude way, subject to important modifications, the reader will be prepared to realize the formative system developed in the construction of our northern hemisphere.

Through a sequence of phases, somewhat distinctively bounded by periods of depression and consequent submergence, and periods of elevation and consequent drainage, land was added to an initial nucleus by enormous marine accumulations, the débris of animal organisms, and the detritus from terrestrial abrasion. Chemical action, heat, and molecular transference hardened these layers into stone, and thus the new-made land, though undergoing change from recurring submergence, and through subaërial denudation, yet, to a great extent, resisted removal while it contributed to the growth of the incipient continent, and formed the ground upon which new-laid strata were heaped.

In the American Museum of Natural History, seven maps, the work of Professor R. P. Whitfield, have recently been added to the Geological Cabinet, intended to exhibit the growth of the eastern half of the North American Continent from 95° west longitude eastward to its shores. The scheme of their arrangement is the exhibition, by contrasted colors, of the superficial areas where to-day the rocks of the various geological epochs are exposed, beginning with the oldest and rising to the youngest, whereby we seem to seize at critical points the