GENERAL OBSERVATIONS. 1027 



Yet, notwithstanding these sources of change, no good evidence in all 

 Paleozoic time, except near or at its end, has been found in the fossils or 

 the rocks, of zones in the earth's climates or of variations in temperature. 

 North America shows in its large coal formation, as compared with that of 

 Europe, that it had then, as it has now, the moister climate ; and therefore 

 that the system of winds was the same as in Recent time, and hence that 

 the system of oceanic currents was the same. Some difference must have 

 existed, and more in the atmosphere than in the waters ; but it was not 

 enough to modify, as far as has been ascertained, the marine fauna of the 

 globe. Uniformity in climate in the northern hemisphere is favored by 

 unobstructed oceanic currents. 



In the later part of the Permian, or at its close, a cold epoch occurred 

 (page 737). At the same time happened one of the earth's most general 

 exterminations of life. But large continental areas were then rising, and 

 the Antarctic Continent was elevated and greatly extended; so that the 

 elevations may have been the cause of the cold. 



After the close of Paleozoic time, zones become apparent (page 791). 

 But even in the earlier part of the Cretaceous period, Cycads abounded 

 in the northern polar regions, showing only a small decline in mean tem- 

 perature since the Cambrian. After the Middle Cretaceous, a more rapid 

 decline began (page 872) ; but, concordantly, large continental elevations 

 were in progress. The increasing elevations during the later Tertiary cul- 

 minated in the Glacial period of the Quaternary. 



Thus, throughout the earth's history since life began, the only cold 

 epochs of which proof has been found occurred near or at the close of the 

 Permian, at the close of the Triassic, and during the Glacial period. At the 

 close of the Cretaceous, another epoch is suspected to have occurred, but 

 without direct evidence. 



The post-Permian and Glacial cold occurred at times when the Antarctic 

 Continent had great extent, and therefore when the earth's polar diameter 

 had unusual elongation. Since the Glacial period, the polar lands have 

 again become submerged; but, inasmuch as Greenland affords evidence of 

 continued subsidence, it may be questioned whether a time of minimum for 

 the polar diameter is yet reached. 



This review of the extremely slow decline of temperature a the earth's 

 climates during its lifetime, — be it 10 millions of years or 600 times this, — 

 with traces of only three or four epochs of cold in the course of the millions, 

 is calculated to give the impression that the eccentricity cycle in the earth's, 

 orbit is a very ineffectual epoch-making agency. 



THE EARTH'S DEVELOPMENT. 



The evolution of the earth's continents and their surface features is one 

 of the two great subjects in the science of Geology. The idea — Continents 

 always Continents — announced by the author first in 1846, has been affirmed 



