Page:Popular Science Monthly Volume 76.djvu/487

Rh widely distributed around the North Pacific. It has also been found as far south as Peru, on one side, and down to the equatorial part of the Indian Ocean on the other. This wide dispersion does not necessarily mean a lowering of the oceanic temperature during this epoch, for this species may have lived in deep water, and therefore could easily find uniform temperature from the equator to the Arctic region. But the sudden change of facies and impoverishment of the fauna over such an enormous area are suggestive. A slight drop in temperature, below 68° F. would account for it.

The last epoch of the Triassic, the Rhætic, has no marine faunas anywhere in America, but the flora, with its abundant cycads, is widely distributed in both the northern and the southern hemisphere. Coal deposits are common in this epoch, and this points to a very uniform and mild climate far beyond the present temperate zones.

At the opening of the Jurassic period we find a Mediterranean marine fauna established in western America; this same fauna also extended from the equatorial regions to Alaska, so that we are without evidence as to climatic zones, and can only infer that the temperature was uniform.

In the Middle Jurassic reef-building corals lived in the waters of the Great Basin Sea, and their remains are quite common in Plumas County, California, but in that province they formed no reefs, for the waters were not clear, and much disturbed by the deposition of volcanic ash. Abundant cycads lived on the land in California at this time, adding their testimony to the warmth of the climate. This same Middle Jurassic marine fauna extended up to Queen Charlotte Islands, and to southern Alaska, in the latter place with cycads interbedded with the salt-water fossils. Here, as was often the case, the cycads extended some distance north of the corals, a coral reef with Astræidæ being known in this epoch on Queen Charlotte Islands, in 53° N. lat., while cycads occur as far north as 57° N. lat. In this same epoch the northern limit for coral reefs in the Atlantic region was 53° N., in southern England, while the other invertebrates and cycads ranged up to 80° N. lat. A mild climate must have extended up nearly to the pole.

The Upper Jurassic of California shows a sharp contrast to the preceding epoch; its marine fauna is scanty, and what little there is belongs to the boreal type, the Aucella fauna, which is characteristic of Russia, northern Siberia and Alaska. For a short time this fauna ranged down into the edge of the tropics in Mexico. This does not mean that the climate was cold, but merely that the temperature was lower than that at which reef-building corals and the other sensitive invertebrates could flourish. In the Lower Cretaceous we find the same boreal type still persisting as far south as middle California. But here,