12 UNIFORMITARIANISM AND ACTUALISM 



catastrojDhic events of our old, our very old earth, come about at a 

 snail's pace. Most geologic changes occur at a pace which can be 

 estimated at something between 1 mm per century and 1 mm per 

 year. In this view, most apparent catastrophes in the history of the 

 earth can be explained by normal processes, actually at work on 

 earth. By slow crustal movements, by slow climatic changes, by 

 erosion and sedimentation, by a repetition of volcanic events or 

 earthquakes. 



VARIATIONS IN INTENSITY OF PROCESSES: THE PULSE OF THE EARTH 



It is not necessary that these actual processes have always been 

 uniform in intensity although they were uniform in kind. To take an 

 example, we know of several Ice Ages in the recent and in the more 

 early geological past. Although an ice-cap extending over most of 

 northwestern Europe and North America is quite a different pro- 

 position from the small vestiges of ice-caps now still remaining on 

 Greenland, Iceland and Spitzbergen, the processes of ice-cap forma- 

 tion are not different. There is a difference in extension, or size, only. 

 The local climatic conditions, the surplus of snow fall, the flowing- 

 out of the ice towards the rim of the ice-cap, the formation of 

 moraines, everything one can think of, arc the same in both cases. 

 With the actual processes at work in Greenland now, we can interpret 

 the much larger ice-caps which covered northwestern Europe and 

 North America in the recent past. The processes are similar, although 

 of varying intensity. 



Conversely, when we find fossils of tropical plants or ancient 

 Dinosaur tracks on Greenland and Spitzbergen, in sediments older 

 than the Ice Age, we conclude that a tropical climate then existed 

 that far north. Not from a sudden catastrophic change of, say, the 

 amount of radiation received from the sun, but through slow changes 

 for which very minor variations in intensity of one or a number of 

 processes now, as before, in operation can be made responsible. 



To a geologist, consequently, everything is in motion. We speak 

 of "our unstable Earth" or of "our moving Earth". But not only 

 this; we also recognize definite periods of a slower or a quicker 

 tempo of certain processes. This has been most concisely expressed 

 in the title of Professor Umbgrove's textbook The Pulse of the Earth. 

 But even such a pulse-beat is, as we saw, not due to catastrophes 

 but only to small changes in the intensity of slow actual processes. 



