258 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



remembered that when we speak of cool chmates we do not mean that the chmate 

 became cooler merely among the uplifted mountains. Of course the fact that a given 

 region was uplifted necessarily made it cooler than formerly, but the geological record 

 preserves little evidence of this. The subaerial formations which have come down from 

 those early times were almost wholly deposited at low altitudes, for otherwise they could 

 not have been preserved. Many were manifestly laid down near sea-level, for they are 

 interstratified with marine deposits. Moreover, a large part of the evidence as to the 

 climate of ancient geological times comes from marine fossils. Accordingly, when we 

 speak of periods of cool and cold climates, we refer to conditions at sea-level. It may be 

 that pronounced crustal deformation would cause the earth's climate to become cool even 

 at sea-level, because of changes in oceanic and atmospheric circulation. Possibly also the 

 reduction of the amount of aqueous vapor in the air by reason of an increase in the size 

 of the continents would add to this effect. Yet, according to the law of chances, mountain- 

 making, and especially the upheaval of continents, ought often to cause vast oceanic areas 

 to become warmer than hitherto instead of colder; for cold currents would be prevented 

 from reaching low latitudes as often as they would be permitted to reach them, and warm 

 currents would be similarly affected whenever barriers were interposed. Therefore we 

 ought in many cases to find that periods of mountain-making and continental uplift are 

 followed by periods of warmth over a considerable portion of the earth. This result 

 might be less marked than a cooling effect, for any increase in the number of land barriers 

 would tend to isolate certain polar portions of the ocean and to prevent them from being 

 warmed by currents from the equator. Nevertheless we should scarcely expect to find so 

 preponderating a tendency toward cool conditions, even in low latitudes, whenever pro- 

 nounced movements of the earth's crust take place. Hence some other cause of climatic 

 variation seems necessary. Moreover, such movements can scarcely account for the com- 

 paratively rapid succession of cold glacial and warm, or arid, interglacial epochs, a fact 

 which Professor Schuchert takes care to indicate. Hence, from this point of view also, 

 some other cause seems needed. 



After discussing the relation of mountain-making and climate. Professor Schuchert 

 takes up the new volcanic theory of climate and tests it by the geological record. He shows 

 that although mountain-making and cool climates are usually associated, there appears 

 to be no correspondingly close association between cool climates and volcanism. This is 

 especially noticeable at the end of the Mesozoic and during the Eocene, when the greatest 

 known volcanic activity of geological times does not appear to have produced any marked 

 glaciation. Moreover, the Permic and Pleistocene glaciations seem not to have been 

 coincident with periods of exceptional volcanic activity, but followed them at intervals 

 which, although not of great length geologically, must have been measured in hundreds of 

 thousands of years. This is quite inconsistent with the volcanic theory, for there is no 

 reason to think that even the finest volcanic dust remains in the atmosphere more than 

 a few years. Accordingly unless further study shall disclose unexpected evidence of 

 widespread volcanic activity coincident with glaciation, it seems wise to accept Professor 

 Schuchert's conclusion that: "Volcanic dust in the isothermal region of the earth does not 

 appear to be a primary factor in bringing on glacial climates. On the other hand, it can 

 not be denied that such periodically formed blankets against the sun's radiation may have 

 assisted in cooling the climates during some of the periods when the continents were highly 

 emergent." 



This conclusion, based on the whole extent of geological time, is almost identical with 

 that which we have previously reached from a study of the short period of thirty years 

 since careful measurements of solar radiation were begun, soon after 1880. 



One other theory receives attention at the hands of Professor Schuchert, and here again 

 his conclusion, based on a vast lapse of time, agrees w4th that which we have already 



