The Fiold of Geology— McGee. 199 



ous testimony recorded in the crumpled strata of the mountains. 

 Yet substantial progress is m^de durin;^ each decade in the subju- 

 gation of this refractory territory. 



Within about a decade an inference of the highest moment 

 has been made by American geologists concerning diastatic move- 

 ment, viz: that certain orogenic movements are consequent upon 

 gradational transfer of matter; that the earth is in a condition of 

 isostacy, and that as the rains bear detritus from the mountains 

 into the seas, the unloaded mountains rise and the loaded sea bot- 

 toms sink; and thus that a part of the deformation of the outer 

 shell of the earth is consequent upon the processes of gradation. 

 At first blush it might appear that the great problem of earth 

 movement is solved by this discovery; but consideration shows that 

 these consequent diastatic movements are but the indirect result of 

 antecedent diastatic movements for which no adequate cause has 

 yet been assigned. It is evident that if diastatic movement were de- 

 pendent solely upon transfer of sediments it would progressively 

 diminish with lapse of time, that the mechanism of mountain 

 building and continent growth would soon be clogged by increas- 

 ing friction, and that the terrestrial surface would be quickly 

 graded so completely that further movement would cease; but the 

 rocks record diastatic activity throughout geologic time, now in- 

 creasing, now diminishing, but on the average probably increasing 

 rather than diminishing, and perhaps as potent to-day as during 

 any past time. . So deformation is separable into two classes of 

 movements, that depending upon transfer of sediments, which may 

 be designated consequent; and that for which cause has not yet 

 been assigned (unless the "contraction" theory be accepted), which 

 may be called antecedent. Discriminated upon a different basis 

 they fall into two classes approximately but not exactly coinciding 

 with these, namely, orogenic^ or mountain making movements, 

 and epeirogenic^* or continent building movements. 



The first of these classes of diastatic movement may be 

 set aside as at least partially explained, though many details 

 remain to be elaborated; and this part of the field is yet prom- 

 ising to the student. But it is the remaining portion of the 

 field of geology defined by deformation which, above all oth- 

 ers, appears to afford promise for the future, and especially to 

 the systematic student who seeks to dig deeper than his fellows; 



*A tr-rm proposed by (lilhort. 



