256 



THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AlVIERICA. 



the sea. The contmental glaciers of Pleistocene time were located mainly in the northern 

 portion of the northern hemisphere, while those of Permic time reached their greatest 

 extent 20° to 40° south of the present equator, and to a less degree between 20° and 40° 

 north of the equator. The Pleistocene glaciation was general in the arctic region, while 

 that of Permic times almost certainly did not prevail in that region. Both periods of 

 glaciation apparently consisted of a series of glacial and interglacial epochs, as is clearly 

 brought out by Professor Schuchert. The evidence consists in part of an abundant 

 interglacial flora which is found in many cases between distinctly glacial deposits during 

 Pleistocene glacial times, and of coal beds which, in Australia, are interstratified with 

 glacial till of Permic age. These two things are typical of a great body of evidence which 

 indicates that glaciation did not last uninterruptedly throughout either period. _ The 

 cUmate apparently fluctuated back and forth between conditions which promoted glaciation 

 and those which caused the ice to reth-e. 



A rapidly growing body of evidence indicates that, in addition to the well-known Pleisto- 

 cene and Permic periods of glaciation, there were at least two and probably three other 

 periods not merely of local but widespread glacial climates. All of these were geologically 

 very ancient and were earlier than the Paleozoic era. The last of them was at or near the 

 close of Proterozoic time; another was still earlier, although its position is somewhat 

 doubtful; the third was at the very beginning of Proterozoic time and almost at the begin- 

 ning of earth liistory as known to geologists. One at least of these periods appears to have 



Fio. 86.— Geologic.ll Changes of Climate and Movements of the Earth's Crust. (After Schuchert.) 



consisted of more than one epoch, for red deposits, apparently indicative of aridity or 

 warmth, appear between glacial deposits. The number of glacial and interglacial epochs 

 in this period may be considerable, although as yet our knowledge is incomplete. In the 

 other two glacial periods of the Proterozoic era the evidence is as yet so slight that we can 

 not tell whether they consisted of merely one epoch or of many. 



Not even yet, however, as Professor Schuchert goes on to say, is the physical evidence 

 of former glacial climates exhausted, for the notable Table Mountain tilUtes of South Africa 

 point to a cold climate that occurred at least locally late in Siluric times. Finally, there 

 may have been a seventh cool period in Liassic, that is, early Jurassic time, but the biologic 

 knowledge so far at hand indicates that it was the least significant among the seven probably 

 cool to cold chmates so far discovered in the geological record. In addition to the seven 



