The Field of Geology— McGee. 195 



simple geoidal form, or heteromorpJiism. Play is thus given to the 

 operations of the second principal and the last four subordinate cat- 

 egories, which are also intimately related and combined and tend to 

 produce heterogeneity in the external shell of the earth. The joint 

 result is differentiation of the earth's exterior — the antithesis of 

 those processes of concentration and segregation by which the planet 

 was originally formed . The passage from the stage of segrega- 

 tion to that of differentiation represents the senescence of the planet; 

 these stages define the provinces of the astronomer and geologist 

 respectively; and the latter defines in like manner that portion of 

 the field of knowledge within which the inductive method of rea- 

 soning is alone applicable, by reason of the ever increasing com- 

 plexity of the phenomena. 



The domain of geology being thus outlined, it remains to indi- 

 cate those fields which have been well covered by investigation 

 and those which yet remain untrodden; and the first principal cat- 

 egory, which defines the least-known field, may be passed for the 

 present. 



Geology found birbh with the study of the sedimentary rocks 

 and their contained fossils; the extravasated rocks soon after re- 

 ceived attention, and at a comparatively early period the metamor- 

 phic strata and other rocks produced by the alteration of sediments 

 and extravasated materials, etc., came under investigatpn; and so 

 the lines of pioneer research were directed toward the genetic 

 classification of rocks. Great progress has been made along these 

 lines, and most of the rocks of the explored earth have been clas- 

 sified with a greater or less degree of refinement. The sedimentary 

 strata are generally classified either by their own sequence or by 

 the degree of biotic development of their contained fossils, or chron- 

 ologically; some of the extravasated rocks and many of those pro- 

 duced by alteration are simply classified by their extrinsic charac- 

 ters, or petrographically ; while certain other rocks of both kinds 

 are classified by their constituents, or mineralogically. The clastic 

 rocks — the products of deposition — especially have received careful 

 scrutiny; out of their study has grown the greater part of geo- 

 logic literature; surveys and commissions have been endowed for the 

 purpose of investigating them; national and international con- 

 ventions have been established to discuss them ; and then* relation 

 to the arts and to the welfare of the race have been pointed out re- 

 peatedly. This field of geology has been carefully covered; and 



