Page:Popular Science Monthly Volume 80.djvu/399

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ANCIENT PORTALS OF THE EARTH

teaus and oceanic troughs dates from the Permo-Carboniferous revolution, although the North Atlantic and the South Atlantic were not occupied by the sea until early Tertiary time.

During Cambrian time the Bering, Central American, Iberian, and Asia Minor portals were open, probably corresponding to regions of Pre-Cambrian folding. The others had not yet come into existence, or at any rate information is lacking concerning them. In the Lower Silurian we know of the existence of the Bering portal. During the Devonian the Bering and the Asia Minor portals were open all the time, the Crimean part of the time, while the Bokharan portal was merely part of the open Asiatic sea. During the Carboniferous era all the major portals were open at times except that leading down to Madagascar. Somewhere near the border between Coal Measures and Permian the Paleozoic topographic revolution inaugurated centers of distribution and portals connecting them that held sway during nearly the whole of Mesozoic time.

If diastrophism should be the final arbiter of the division of geological time, then on the basis of physiography the Permian, including the Artinsk stage, should be included in the Mesozoic. For while Permian life is distinctly more closely related to that of the Paleozoic, Permian physiography is like that of the Triassic.

Interregional Faunal Zones.—It is customary to speak of cosmopolitan faunas in the past ages, but this term is properly applicable only to the Cambrian and Silurian, when local differentiation had not yet caused the extremes of later times. And even in the Cambrian, as the scanty faunas become better known, provincial differences appear. In Devonian time provincial distinctions were already well developed, and the invasion of a region by an exotic fauna is easily recognized. The first interregional migration that is definitely known occurred early in the Upper Devonian, when the American waters were invaded by a fauna that could not have sprung from its predecessors in that region, but was endemic in Eurasia. This is the zone of the Cuboides fauna, which was followed by still further immigration from the same center of dispersion, in the zone of Manticoceras intumescens of the Upper Devonian. The connection with Europe was through the back door, through northern Siberia, across North America, for the continental mass of North Atlantis and Appalachia prevented direct communication.

With the opening of the Carboniferous age the subsidence of the southern part of Appalachia allowed direct intermigration between the waters of western Europe and the Mississippian Sea through the Poseidon basin. Here we find the faunal zone of Aganides rotatorius common to the two regions, but unknown anywhere else. This is the first direct invasion of the American seas by a population from the western Tethys, or ancient Mediterranean basin. In the latter part of