Geology and Mineralogy. 565 



period are correlated with the record of diastrophism, and the 

 whole is put together on p. 285 into a diagram which shows the 

 curves of coal formation, limestone deposition, aridity, tempera- 

 ture, and diastrophism. Schuchert concludes that the major 

 cause in the climatic changes through geologic time is to be 

 found in the topographic changes of the earth's crust from period 

 to period. There are other causes, perhaps many others, but 

 they are subordinated in importance to diastrophism. 



This description of the contents of the volume serves to show 

 its importance to many lines of investigation. It gives a firmer 

 basis to the subject of climatic changes through historic time 

 than had previously appeared. Many meteorologists and histo- 

 rians have insisted that there w T as no evidence of climatic change, 

 that there was merely an irregular variability from year to year, 

 and that the ruin of ancient empires or the migration of barba- 

 rian hordes were of purely human initiation, not the human 

 response to the compulsion of nature. Since both factors would 

 enter as causes most of the evidence could be interpreted in 

 two ways, either for or against climatic change and would not 

 compel a view contrary to that which had been previously 

 believed. Here, however, three independent lines of evidence 

 are assembled w T hich have no causal connection with the human 

 factor. They are lake strands, river terraces, and tree growth. 

 Together they lead to the belief that there have been changes 

 which have had considerable effect upon man in those regions 

 where the supply of moisture is the critical factor in the climate, 

 that is, especially in the arid and semi-arid regions. The changes 

 in mean annual temperature do not seem to have been so material. 



Other scientists have held that climatic changes during historic 

 times have taken place but that they were slow and progressive, 

 a part of a drying-up process extending through geologic time 

 and leading ultimately toward lessened oceans and widened 

 deserts. The evidence brought forward in this book shows that 

 such a view has no support. 



Huntington's work has gone far toward establishing an inter- 

 mediate view and yet one which has features not found in either. 

 Climate is subject to pulsatory change ; smaller and larger rhythms 

 are superposed. The changes may go forward very rapidly, so 

 that the population of an arid or semi-arid region may be largely 

 reduced in a single generation. The changes in the past 3,000 

 years have been small in comparison with the changes which 

 have taken place since the culmination of the last glaciation and 

 yet they are to be regarded as of the same nature, the continua- 

 tion of the geologic past into the present. 



From the standpoint of the geologist the contributions of 

 Huntington as well as Schuchert are of importance ; for as Lyell 

 showed that the present is the key to the past in the crustal his- 

 tory of the earth, in a similar manner the key to the climatic 

 history is to be found in the study of the present climates and 

 their fluctuations. 



