286 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



region is of this time. If so, these occurrences record a distribution of glacial materials 

 very similar to that of Permic time. Again, the Proterozoic tillites of Africa are clearly 

 of another age, so that there is evidence of at least three periods of glaciation previous to 

 the Paleozoic. 



The physical evidence of former glacial climates is even yet not exliausted, for the Table 

 Mountain tilUtes of South Africa point to a cold cUmate that apparently occurred, at least 

 locally, late in Siluric time. Finally, there may have been a seventh cool period in early 

 Jurassic time (Lias), but the biologic evidence so far at hand indicates that it was the least 

 significant among the seven probable cool to cold climates so far discovered in the geologic 

 record. 



The data at hand show that the earth since the beginning of geologic history has peri- 

 odically undergone more or less widespread glaciation and that the cold climates have 

 been of short geologic duration. So far as known, there were seven periods of decided 

 temperature changes and of these at least four were glacial climates. The greatest intensity 

 of these reduced temperatures varied between the hemispheres, for in earliest Proterozoic 

 and Pleistocene time it lay in the northern, while in late Proterozoic and Permic time it 

 was more equatorial than boreal. The three other probable periods of cooled climates are 

 as yet too Uttle known to make out their centers of greatest intensity. 



Of the four more or less well-determined glacial periods, at least three (the earliest 

 Proterozoic, Permic, and Pleistocene) occurred during or directly after times of intensive 

 mountain-making, while the fourth (late Proterozoic) apparently also followed a period of 

 elevation. The Table Mountain tilhtes of South Africa, if correctly correlated, fall in with 

 the time of the making of the great Caledonian IVIountains in the northern hemisphere. 

 On the other hand, the very marked and world-wide mountain-making period, with 

 decided volcanic activity, during late Mesozoic and earliest Eocene times, was not accom- 

 panied by a glacial cUmate, but only bj^ a cooled one. The cooled period of the Liassic also 

 followed a mountain-making period, that of late Triassic time. We may therefore state that 

 cooled and cold chmates, as a rule, occur during or inmiediately follow periods of marked 

 mountain-making — a conclusion also arrived at independently by Ramsay (1910: 27). 



Geologists are begiimiing to see clearly that the lands have been periodically flooded 

 by the oceans, and the times of maximum submergence and emergence of the continents 

 since earUest Paleozoic time are fairly well known. The two marked glacial periods since 

 Cambric time (Permic and Pleistocene) and the three other more or less cooled chmates 

 (late Siluric, Liassic, and late Cretacic) all fall in with the times when the continents were 

 more or less extensively and highly emergent. There were no cold chmates when the 

 continents were flooded by the oceans, and it may be added that the periods of widespread 

 limestone-making preceded and followed, but did not accompany, the reduced climates. 

 On the other hand, the periods of greatest coal-making (Upper Carbonic and Upper Cre- 

 tacic) accompanied the time of greatest continental flooding and preceded the appearance 

 of cooled climates. 



The more or less coarse red sediments seen at many horizons of the geologic column 

 are interpreted as the deposits of variably arid climates, or those that are alternately wet 

 and dry. In the Paleozoic they are seen more often at the close of the periods when the 

 seas were temporarily withdrawn and the lands were most extensive. These red deposits 

 alternate with formations that are either wholly marine or of brackish-water origin, and 

 in the latter case of gray, green, blue, or black color. 



Humphreys has shown that volcanic dust in the isothermal region of the earth's atmo- 

 sphere does appreciably r^educe the temperature at the surface of the globe. It is thought 

 that if explosive volcanoes continued active through a more or less long geologic time, this 

 factor alone would bring on, or largely assist in bringing on, a more reduced temperature 

 or even a glacial climate. If then, we may further postulate that volcanic activity is 



