234 EARTH-CRUST MOVEMENTS AND THEIR CAUSES. 



stood as to their laws and their causes. While the causes of the phe- 

 nomeua of the second group, hidden forever from direct observation in 

 the inaccessible depths of the earth's interior, are still very obscure; 

 and yet partly on account of this very obscurity, but mainly on account 

 of their fundamental importance, it is just these which are the most 

 fascinating - to the geologist. The former group, constituting, as it does, 

 the terrestrial drama enacted by the suu, its interest is shared by 

 geology equally with other departments of science, such as physics, 

 chemistry, and biology. The phenomena of the second group are more 

 distinctively the field of geology. 



Tf we compare the earth with an organism then these interior forces 

 coustitut e its life force, while the other group may be likened to the 

 physical environments against which it eternally struggles, and the 

 outcome of this struggle determines the course of the evolution of 

 the whole. Now in biological science nearly the whole advance has 

 heretofore been by study of the external and more easily understood 

 phenomena, thus clearing the ground and gathering material for attack 

 on the interior fortress, and the next great advance must be through 

 better knowledge of the vital forces themselves. The same is true of 

 geology. Nearly all the progress has heretofore been by the study of 

 the exterior phenomena, such as erosion, transportation, sedimentation, 

 stratification, distribution of organic forms in space, and their succes- 

 sion in time, etc. Many of the laws of these phenomena have already 

 been outlined, and progress to-day is mainly in filling in and complet- 

 ing this outline; but the next great step must be through a better 

 knowledge of the interior forces. This is just what geological science 

 is waiting for to-day. Now the first step in this direction is a clear 

 statement of the problems to be solved. The object of this address is 

 to contribute something, however small, to such clear statement. 



EFFECTS OF INTERIOR FORCES. 



As the interior of the earth is inaccessible to direct observation, we 

 can reason concerning interior forces only by observation of their effects 

 on the. surface. Now these effects, as usually treated, are of three main 

 kinds: (1) Volcanoes, including all eruptions of material from the inte- 

 rior; (2) earthquakes, including all sensible movements, great and 

 small; (3) gradual small movements affecting large areas, imperceptible 

 to the senses, but accumulating through indefinite time. 



It is certain that of these three the last is by far the most funda- 

 mental and important, being, indeed, the cause of the other two. Vol- 

 canoes and earthquakes, although so striking arid conspicuous, are 

 probably but occasional accidents in the slow march of these grander 

 movements. It is only of these last, therefore, that we shall now speak. 



KINDS AND GRADES OF EARTH-CRUST MOVEMENTS. 



The movements of the earth's crust determined by interior forces are 

 of four orders of greatness: (1) Those greatest, most extensive, and 



