A CENTURY OF GEOLOGY. 281 



t'riftail jx'i'lods 1)1 tJh- Ji/sfory of tite earth. — Such pcM'iods of rapid 

 change may '.veil )>e called critical periods or revolutions. Thev are 

 marked bv several characteristics: (1) By widespread oscillations of the 

 earth's crust, and therefore by almost universal unconformities. (2) 

 By widespread changes of physical geography, and therefore by great 

 changes in climate. (3) By great and widespread changes in oiganic 

 forms, produced partly by the physical changes and partly by the 

 extensive migrations. (4) By the evolution of new dominant types, 

 which are also the cause of extensive 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 con- 

 spicuous features. 



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

 greatest change: (1) The pre-Cambrian or Laurentian 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 mainl}^ 

 advocated by the writer of this sketch, but I believe that, with per- 

 haps 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 uni(iue 

 phenomena of the Glacial epoch. 



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

 terious 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 char- 

 acteristic marks of glacial action, and with the fact that Alpine glaciers 

 were far more extensive formerly than now, and had, moreover, con- 

 ceived 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 l)oldly announced that the whol(> 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 everywhere, 

 and again astonished the world by asserting that the whole northern 

 part of the North American continent was modeled liy a moving ice 



iCritic'al Periods, etc., Auierican Journal of Scienee, Vol. XIV, p. 99, 1877; Bul- 

 letin of the Geological Department of the University of California, Vol. I, No. 11, 189o. 



