284 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



as poplars, birches, beeches, oaks, elms, magnolias, limes, and maples. The swamp cypress, 

 Nathorst says, "formed forests, as in the swamps in the southern portion of the United 

 States. This conclusion is also confirmed by the occurrence of the remains of rather 

 numerous insects" (1912: 341). All of the plants mentioned then flourished as far north 

 as 79° N. latitude, and even at nearly 82° in Grinnell Land. This is evidence that in early 

 Miocene time the climate was at least warm-temperate in Arctic America. 



Again, Dall (1895) states that in Middle Miocene time considerable reduction of the 

 climate appeared, for the Atlantic Chesapeake faunas were those of temperate waters and 

 they spread southward as far as the eastern area of the Gulf of Mexico. Similar conditions 

 are noted by the same conchologist in the northern Pacific Ocean. He says: 



"The conditions indicated by the faunas of the post-Eocene Tertiary on the Pacific Coast from 

 Oregon northward are a cool temperate climate in the early and Middle Miocene, a warming up 

 toward the end of the Miocene culminating in a decidedly more warm-water fauna in the Pliocene, 

 and a return to cold if not practically Arctic temperatures in the Pleistocene" (1907: 457-8). 



The Tertiary was an era of extraordinary crustal movements, finally resulting in the 

 greatest mountain chains of all geologic time. These movements began in early Eocene 

 time in the Rocky Mountains and at the close of tliis epoch further deformation took place 

 in the Klamath and Coast Ranges of Oregon and the Santa Cruz Mountains of California. 

 In Europe the elevations of Tertiary time started at the close of the Eocene in the Pyrenees, 

 and in the Miocene the entire "Alpine system" was in elevation. This um-est spread at 

 the same time to the Caucasus, Asia, and to the entire Himalayan region of highest moun- 

 tains and elevated plateaus, an area 22° of latitude in width. It is probable that all of the 

 world's great mountain chains were more or less reelevated in Miocene and Phocene times, 

 resulting in the present abnormally high stand of the continents when contrasted with the 

 oceanic mean level. 



These elevations also altered the continental connections, for North and South America 

 were reunited in Miocene times, and western Europe, Greenland, and America were severed 

 late in the Tertiary era, the exact time being as yet not clearly established. With these 

 great changes also must have come about marked alterations in the oceanic currents and, 

 as a consequence, in the distribution of heat and moisture over vast areas of the nor- 

 thern Atlantic lands. It is admitted by all paleontologists that the marine waters of 

 late Phocene times in the Arctic region were cool, and the widespread glacial tills of the 

 northern hemisphere are evidence of a glacial climate of varying intensity throughout 

 Pleistocene time. 



CONCLUSIONS. 



Our studies of the paleometeorology* of the earth are summed up in figure 90. We 

 have seen that two marked glacial periods are clearly estabhshed. The one best known 

 was of Pleistocene time and the other, less well known in detail, of earUest Permic 

 time. Both were world-wide in then* effects, reducing the mean temperatures sufficiently 

 to allow of vast accumulations of snow and ice, not only at high altitudes, but even more 

 markedly at low levels, with the glaciers in many places attaining the sea. We also learn 

 that the continental glaciers of Pleistocene time were dominant in the polar regions, 

 while those of Permic time had their greatest spread from 20° to 40° south of the present 

 equator, and to a far less extent between 20° and 40° in the other hemisphere. There is 

 also some evidence of glaciers in equatorial Africa in Permic time. We may further state 

 that, although Pleistocene glaciation was general in the Arctic region, there certainly was 

 none at this pole in early Permic time, because of the widespread and abundant marine 

 faunas that are not markedlj^ unlike those of the Upper Carbonic; as for the south pole, 

 our knowledge of pre-Pleistocene glaciation is as yet a blank. 



* H. F. Osborn, Compte Rendu, Congrcs Inteiuat. Zool., Berne, 1904, 1905: SS. For a review of the papers 

 treating of paleometeorology, see M. Semper, Geol. Rundschau, I, 1910-. 57-80. 



