ANCIENT CLIMATES 483 



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 Bhaetic, 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 uni- 

 form 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 ex- 

 tended 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 Mid- 

 dle 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 ex- 

 tended some distance north of the corals, a coral reef with Astraeidas 

 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, 



