THE CAUSE OF THE GLACIAL PERIOD. 515 



plication, uplifts, and faults in the birth and growth of mount- 

 ains. Further, it is implied also that stress in the earth's crust 

 had been gradually increasing through long previous time, 

 while the processes of mountain-building failed to keep pace 

 with contraction, but were still sufficient to keep the earth's 

 deformation less than is required to produce glaciation ; for no 

 evidences of intense and widely extended glacial conditions are 

 found in the great series of Tertiary and Mesozoic formations, 

 representing the earth's history through probably ten or fifteen 

 millions of years. And indeed these conclusions, drawn from 

 the Quaternary Glacial period and the absence of glaciation 

 through vast eras preceding, accord well with the known age 

 and stages of growth of mountain-ranges that have been formed 

 during these eras. No epoch since the close of Palaeozoic time 

 has been more characterized by mountain-building than the 

 comparatively short Quaternary, whose duration may probably 

 be included within one hundred thousand or two hundred 

 thousand years. The continuation of the earth's faunas and 

 floras, with only very slight changes of species and exceedingly 

 rare instances of extinction through the Quaternary period, not- 

 withstanding its remarkable vicissitudes of climate and changes 

 in the relative heights of land and sea, which seem especially 

 adapted to produce both modifications and extinctions of or- 

 ganic forms, bears indisputable testimony of the brevity of this 

 period, when compared with those of Tertiary and Mesozoic 

 time. As we extend our investigations backward in the geo- 

 logic record, the species now existing are found in decreasing 

 numbers until we come to the beginning of the oldest. In 

 their places very different species, genera, orders, and groups 

 tenanted the earth before them ; and the gradual and doubtless 

 very slow evolution of the present from the past must have re- 

 quired duration almost incommensurable by years and centu- 

 ries. But the total of mountain-building that has taken place 

 during the Mesozoic and Tertiary eras is disproportionately 

 small in comparison with that of the Quaternary period, even 

 when ample allowance is made for long and very great denu- 

 dation. 



Not until we go back to the Permian and Carboniferous 

 period^ are numerous and widely distributed proofs of very 



