Page:Popular Science Monthly Volume 80.djvu/398

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this, since ancient continents can be outlined only by the distribution of the faunas and floras of the land, and ancient seas can be traced only in the remains of petrified inhabitants of the waters.

In the Book of Genesis we read: "And God said, Let the waters under the heaven be gathered together into one place, and let the dry land appear; and it was so. And God called the dry land Earth; and the gathering together of the waters called He seas: and God saw that it was good." This is certainly the shortest account of the origin of continents and seas, and probably as good as any. But we do not any longer think that it all happened at one time.

Since we can know definitely the geology of only that part of the earth which is now land, and since we know the underlying strata of only a small part of that, we can only conjecture concerning the history of regions now buried under the oceans. No one man, nor group of men, is qualified to make a dogmatic statement as to the origin of continents and seas, such as that quoted above from Holy Writ. There is ample room for differences of opinion, starting from the same facts. Consequently, geologists and physical geographers are divided into two camps. One holds that the major divisions of land and sea were always as they now are; this is the doctrine of permanence of continental plateaus and oceanic basins. The other group advocates the idea of constant change in the position of land masses and oceanic troughs. To them the grand features of the earth do not bear the marks of hoary antiquity, but are youthful characters, due to rather modern diastrophism of the crust.

The truth probably lies somewhere between the two extremes, and the differences between the two camps consist rather in statement than in fundamental doctrine. Even the most conservative upholders of the theory of permanence admit that some of the continental areas have been covered, in the past, by seas of almost oceanic size and depth. And the most radical advocates of the shifting of lands and seas believe that some of the continental masses have always been continents, and that some of the great depressions have always been oceans.

Further, it becomes plainer, as paleogeographic studies go deeper into the history of the earth, that the dominant ancient features are not obliterated by later changes, but are merely obscured. Continents that were dismembered have been united again; seas that existed in the early days have recurred. Which is to say that whether continental plateaux and oceanic troughs have been permanent or not, the regions of diastrophism have been permanent, that when crumpling and dislocation of the earth's crust have once started, they have kept up with recurrent activity all through the succeeding ages.

The ancient portals all lie in regions where the Tertiary mountain folds touch the great lines of crumpling begun in the late Paleozoic topographic revolution. The present distribution of continental pla-