114 J. LE CONTE — EARTH-CRUST MOVEMENTS AND THEIR CAUSES. 



agencies, together with all other terrestrial phenomena which constitute 

 the material of science; in the other group, igneous agencies and their 

 phenomena alone. The forces in the one group are exterior; in the 

 other, interior ; in the one, sun-derived ; in the other, earth-derived. 

 The one forms, the other sculptures, the earth's features ; the one rough- 

 hews, the other shapes. The general effect of the one is to increase the 

 inequalities of the earth's surface, the other to decrease and finally to 

 destroy them. The configuration of the earth's surface, the distribution 

 of land and water — in a word, all that constitutes physical geography 

 at any geological time is determined by the state of balance between 

 these two eternally antagonistic forces. 



PHENOMENA TO BE STUDIED. 



Now the phenomena of the first group, lying as they do on the surface 

 and subject to direct observation, are comparatively well understood as 

 to their laws and their causes. While the causes of the phenomena of 

 the second group, hidden forever from direct observation in the inacces- 

 sible depths of the earth's interior, are still very obscure ; and yet partly 

 on account of this very obscurhty, but mainly on account of their funda- 

 mental 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 sun, 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. 



If we compare the earth with an organism, then these interior forces 

 constitute its life-force, while the other group may be likened to the 

 physical environments against which it eternally struggles, and the out- 

 come 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 inte- 

 rior fortress, and the next great advance must be through better knowl- 

 edge of the vital forces themselves. The same is true of geology. Nearly 

 all the progress has heretofore been by the study of the exterior phe- 

 nomena, such as erosion, transportation, sedimentation, stratification, dis- 

 tribution of organic forms in space and their succession in time, etcetera. 

 Many of the laws of these phenomena have already been outlined, and 

 progress today is mainly in filling in and completing 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 today. Now 



