Page:Popular Science Monthly Volume 53.djvu/37

Rh Indian region rose, and this time to its greatest height, so as to allow the excavation of the very deep valleys and cañons through the then existing table-lands which now form the submarine plateaus. This high elevation was characterized by such deep sculpturing as to give rise to some of the boldest physical features of the lands with which we are familiar. With the subsequent depression of the West Indian bridge, the fragmentary islands became much smaller than even now, and upper portions of the drowned valleys were again partly filled with a still newer formation (known as the Columbia of the Southern States). This last has since been elevated a few hundred feet, with some minor changes of level continuing to the present day. The Columbia formation belongs to a mid-Pleistocene epoch. Consequently, the time of greatest elevation and development of the West Indian continent was during the early Pleistocene period—more popularly called the Ice age. These great and recurring changes of level of land and sea, in later geological times, have produced remarkable and startling revolutions in the physical geography of the region, which might seem incredible but for the great array of evidence which is now being accumulated on every hand.

—The elevation of the Antillean bridge is a question of dynamical geology, but the consequent changes of the physical geography naturally affected the distribution of life; accordingly, the biological aspect should be called upon as evidence of the physical changes.

The commingling of the littoral fauna of the Pacific Ocean with that of the Antillean waters confirms the recent separation of the two seas by the elevation of the Central American region.

The character of the deep-sea fauna is very important. The elevation of the West Indies to less than three thousand feet would exclude the Atlantic waters from the Antillean seas, except through two or three shallow straits, and one channel of enormous depth, between the Virgin Islands and St. Croix. According to the late Dr. Brown Goode, the deep-sea fishes belong to modern forms, apparently overflowing through the Caribbean Sea into the Gulf of Mexico. There is no relationship whatever between the deep-sea fishes of these basins and those of the adjacent waters of the Pacific Ocean. The seemingly recent introduction of much the larger proportion of deep-sea fishes from the Atlantic naturally indicates the existence of barriers between the different basins—in other words, an elevation of the region by a few thousand feet. Furthermore, the general modern character of the fish fauna, with the exclusion of the Pacific forms, suggests the inundation of the floors (including possibly a few small sea basins) of the Caribbean and Honduras Seas