550 POPULAR SCIENCE MONTHLY. 



and therefore by great changes in climate. (3) By great and wide- 

 spread changes in organic forms, produced partly by the physical 

 changes and partly by the extensive migrations. (4) By the evo- 

 lution of new dominant types, which are also the cause of exten- 

 sive changes in species. (5) Among the physical changes occurring 

 at these times is the formation of great mountain ranges. The 

 names of these critical periods or revolutions are often taken from 

 the mountain range which form their most conspicuous features. 



There have been at least four of these critical periods, or 

 periods of greatest change: (1) The pre-Cambrian or Laurentide 

 revolution; (2) the post-Paleozoic or Appalachian; (3) the post- 

 Cretaceous or Rocky Mountain; (4) the post-Tertiary or glacial 

 revolution. 



Now, as these critical periods separate the primary divisions of 

 time — the eras — it follows that the Present — the Age of Man — 

 is an era. It may be called the Psychozoic Era. These views 

 have been mainly advocated by the writer of this sketch, but I 

 believe that, with perhaps some modification in statement, they 

 would be accepted by most geologists as a permanent acquisition 

 of science.* 



GEOLOGICAL CLIMATES. 



Attention was first drawn to this subject by the apparently 

 unique phenomena of the Glacial epoch. 



For nearly a century past Alpine glaciers, their structure, their 

 mysterious motion, and their characteristic erosive effects, have 

 excited the keenest interest of scientific men. But until about 

 1840 the interest was purely physical. It was Louis Agassiz who 

 first recognized ice as a great geological agent. He had long been 

 familiar with the characteristic marks of glacial action, and with 

 the fact that Alpine glaciers were far more extensive formerly 

 than now, and had, moreover, conceived the idea of a Glacial epoch 

 — an ice age in the history of the earth. With this idea in his 

 mind, in 1840 he visited England, and found the marks of glaciers 

 all over the higher regions of England and Scotland. He boldly 

 announced that the whole of northern Europe was once covered 

 with a universal ice sheet. A few years later he came to the 

 United States, and found the tracks of glaciers every^^here, and 

 again astonished the world by asserting that the whole, northern 

 part of the ISTorth American continent was modeled by a moving 

 ice sheet. This idea has been confirmed by all subsequent inves- 

 tigation, especially here in America. 



* Critical Period?, etc., American Journal of Science, vol. xiv, p. 99, 1877'; Bulletin of 

 the Geological Department of the University of California, vol. i, No. 11, 1895, 



