322 C. Schuchert — Evolution of Geologic Climates. 



Wliat Knowlton sees so well are the wide-spread fossil 

 floras that usually occur in the middle portion of the geo- 

 logic periods when the transgressions of the warm-water 

 oceans over the continents are greatest; in other words, 

 those times when the lands are smallest and are dom- 

 inated by insular and therefore equable climates due to 

 the wide-spread and warm oceanic waters. But Avhat 

 were the climatic conditions during the early and late 

 parts of the many periods when the continents were larg- 

 est, highest, and most arid? And it is at some of these 

 times when the local or the widely spread tillites were 

 forming ! 



That the oceanic waters were not everywhere and dur- 

 ing most of geologic time equably warm is attested by the 

 varied life distribution seen in the large colonial f oramin- 

 ifers, stony corals, shelled cephalopods, and thick-shelled 

 bivalves (chiefly the cemented forms) and gastropods. 

 We see some or all of these groups of animals connnon in 

 the far north and even in arctic waters during the Silu- 

 rian, Devonian, Pennsylvanian, and Jurassic, but at 

 other times they are either greatly reduced in variety in 

 these regions or almost or completely absent. Take the 

 Cretaceous foraminifers, sponges, corals, and rudistids, 

 and the late (but not latest) Eocene nummulids, and see 

 how they drop out of the record as one proceeds toward 

 the Arctic. In India, nummulites attain to a diameter of 

 eight inches, but in Japan they are all small, and north 

 of Japan there are none of them at all. Moreover, 

 the giants of any stock are rarely seen in far northern 

 waters. These things can only mean temperature dif- 

 ferences, and that the northern waters were frequently 

 under 65° F. And if the oceanic waters were thus vari- 

 able in temperature from time to time and from place to 

 place, we may conclude that the climates of the lands were 

 also varied and had zonal belts. 



On the other hand, w^hen the lands are largest and the 

 marine faunas localized in small sea-ways not widely ac- 

 cessible to the paleontologist, where are the cosmopolitan 

 faunas and the large forms, and where is the variety and 

 abundance of foraminifers, corals, bryozoa, cemented and 

 thick-shelled molluscs, and shelled cephalopods? Ex- 

 ceptions there are, of course, but in general the restricted 

 formations have small faunas and small species that are 

 more or less unrelated to one another. In other words, 

 we have here the stress-assemblages in which a prolifera- 



