GEOLOGY. 231 



ing prog:ress originate with iudividuals, the devices for facilitating in- 

 terchange of thought in modern times are so nnmerons and coiuidete 

 that many conceptions are disseminated with their growth, students 

 keep pace with the progress of tlieir feHows in distant hinds, and so 

 contributions to a general conception may be made by many individuals. 

 The rapid advance of human knowledge witbin recent years must bo 

 attributed not less to the facilities for free interchange of thought now 

 existing than to that ever-increasing liberality of modern students which 

 leads them to share even the tirst fruits of their work with the entire 

 world. 



Geologic science has been enriched by many notable conceptions dur- 

 ing the biennial period 1887-88; some of these are the product of in- 

 dividual minds, while some represent the work of many students upon 

 related or identical problems; but only a few of the more prominent 

 can bo noted. 



GEOLOGIC PHiLosornr. 



Philosophic doctrine is the outcome of thought upon difierent lines; 

 when com])rehensivo it is little affected by the movements upon any 

 one line ; and it is thus so nearly stable that little advance can be per- 

 ceived within a year or even a decade. lUit the time has now come for 

 noting an important step in the development of geologic philosophy; 

 for although the movement began some years since among advanced 

 thinkers, and although it has not yet extended to the text-books or 

 even to the rank and file of workers, its influence is seen in geologic 

 literature and is rapidly extending. 



The jirimary geologic classification was based directly upon the ob- 

 jective phenomena of geology' ; and early geologic literature was per- 

 vaded, and the science shaped, by this fundamental idea. As time went 

 on this classification was found too narrow to represent intelligibly the 

 facts and their relations, and the desire for a more comprehensive tax- 

 onomy was indicated by the semi arbitrary division of the science into 

 various departments in which the minor classes were variously defined 

 an<l grouped: Physical geology; Stru(;tural geology; Stratigraphic 

 geology; Ilistorical geology; etc. Although all such divisions were 

 partly arbitrary, they contained the germ of a, more i)hilosoi)hic classi- 

 fication in which the agencies and conditions of geology are recognized. 

 Progress in this direction culminated in 1884 in a classification devised 

 by Powell to serve as a basis for a bibliogrni)hy of North American ge- 

 ology. The following divisions are recogni/ed in this classitica^^ion : 



5th Ann. Rep. U. S. Geol. Snrvey, 1885, p. xxxiii. 



