INVESTIGATION OF TEMPERATURE CHANGES. 243 



have taken place. The hvnd area correspondhig to each more distant epoch 

 is not only smaller, but it has been exposed to a longer series of geological 

 changes by which the true character of its fossil remains is likely to have 

 been more or less obscured. Thus it is not surprising that it is the more 

 recent formations which, as a rule, furnish the larger portion of the evidence 

 that the climates of various regions have become less warm than they were 

 in former geological periods. And it is not surprising that the most striking 

 evidence of this kind comes from the polar regions ; for while we have no 

 experience of a climate too hot for the development of vegetable life, we do 

 have extensive regions where the cold is too excessive to permit tlie growth 

 of any but the lowest plant forms. Hence the evidence that in the latitudes 

 where now only these low forms occur, there formerly existed an abundant 

 growth of higher ones, such as could only thrive in a much more elevated 

 temperature, is not onl v decisive as to a change of climate, but ujost striking 

 as a fact indicative of greatly altered conditions on the earth's surface ; while, 

 on the other hand, if regions once too hot for anything to grow have now 

 become cooled down sufficiently to allow vegetation to flourish, it would be 

 almost impossible to procure the proof of such a cliange, the earlier condi- 

 tion being only indicated by negative evidence, and going back to a period 

 when marine formations were, as a rule, greatly predominating. 



With these preliminary remarks, we may proceed to a brief statement of 

 some of the leading facts on which is based the assertion that there is ample 

 proof in the records of geological science that, as a whole, the mean tempera- 

 ture of the earth's surface has been diminishing. 



Beginning with the region in which the investigations were made which 

 led to the publication of the present work, we may first call attention to the 

 evidence of a warmer temperature prevailing in the Sierra Nevada during 

 the later Tertiary period. The plants of that region, collected by the Geo- 

 logical Survey of California, have been described by Mr. Lesquereux.* He 

 indicates the results of the investigation, from the climatic point of view, in 

 the following words : '• The plants described here from the Pliocene clearly 

 expose the climate of the period which they represent. Tliey record a tem- 

 perature a few degrees higher, on the average, than that of IMiddle California, 

 or, like the species of the Chalk Bluffs of the Mississippi, they represent a 

 latitude of a few degrees farther south The action of a warmer climate 



* See Report on the Fossil Plants of the Auriferous Gravel Deposits of the Sierra Nevada of California, Mem. 

 Museum Comp. Zoology, Vol. VI. No. 2. 



