322 



SCIENCE. 



[N. S. Vol. V. No. 113. 



Thus, then, all geological agencies are 

 primarily divided into two groups. In the 

 one group came atmospheric, aqueous and 

 organic 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 iu the group are exterior ; 

 in the other, interior ; in the one, sun-de- 

 rived; 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 sur- 

 face, 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 be- 

 tween 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 

 fco 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 inaccessible depths 

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

 and yet partly on account of this very ob- 

 scurity, but mainly on account of their 

 fundamental importance, it is just these 

 which are the most fascinating to the geol- 

 ogist. 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 phe- 

 nomena of the second group are more dis- 

 tinctively the field of geology. 



If we compare the earth with an organ- 

 ism then these interior forces constitute its 

 life-force, while the other group may be 



lilsened 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 ex- 

 ternal and more easily understood phenom- 

 ena, thus clearing the ground and gather- 

 ing 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 here- 

 tofore 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 out- 

 lined, and progress to-day is mainly in fill- 

 ing 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 

 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 INTEKIOR FORCES. 



As the interior of the earth is inaccessible 

 to direct observation, we can reason con- 

 cerning interior forces only by observation 

 of their effects on the surface. Now these 

 efiects, as usually treated, are of three main 

 kinds : (1) Volcanoes, including all erup- 

 tions of material from the interior ; (2) 

 Earthquakes, including all sensible move-^ 

 ments, great and small ; (3) Gradual small 

 movements effecting large areas, imper- 

 ceptible to the senses, but accumulating 

 through indefinite time. 



It is certain that of these three the last is 

 by far the most fundamental and important, 

 being, indeed, the cause of the other two. 



