180 STRUCTURAL GEOLOGY. 



convenient to assemble them here, but I am strongly disposed to accept 

 Mr. Gilbert's conclusions. 



The fourth condition is supposable, but that it is a vera causa I cannot 

 nay. I have often supposed it to be such in the study of particular faults, 

 but on further study it has eluded my apprehension. We sometimes 

 iind a displacement of many thousand feet where the uplifted region is 

 separated from the thrown by a broad zone of gently dipping beds, 

 and it would seem a priori that where two regions were thus separated, 

 the. zone of change would be by flexure rather than by a series' of 

 ruptures, and such is usually though not invariably the case, and this seems 

 to be a question of application of force, or in other words, character of 

 strain, and it cannot be doubted that such a condition must be taken into 

 consideration. Mr. Gilbert- evidently recognizes it as a true cause, for in 

 discussing the Basin Ranges,Jie says: " The displacement of comparatively 

 rigid bodies of strata by vertical or nearly vertical faults, involves little hor- 

 izontal diminution, and suggests the application of vertical pressure from 

 below." 



As to the first cause, rate of displacement, it is manifest that accelera- 

 tion promotes rupture, retardation, flexure; but the other conditions which 

 I have set forth so modify this rule, that rate of displacement cannot with 

 any certainty be determined from facts of flexure and rupture alone. 



The Uinta upheaval was partly by flexing, partly by faulting, and if 

 there were no other conditions to determine these characteristics, we might 

 say that so far as the beds were flexed they give evidence of slow move- 

 ment; and so far as they were ruptured, evidence of a more rapid move- 

 ment; and the twenty -five or thirty -thousand feet of beds which cfcre 

 exposed to view and were involved in this uplift were chiefly of a character 

 quite brittle, and this increases the evidence in favor of #low upheaval by 

 flexing ; but flexing may have been at great depths, and the superincumbent 

 masses produced a condition favorable to flexing even in a rapid rate of 

 movement. Many substances, especially metallic, are known to flow under 

 great pressure, and it is probable that great pressure would produce in all 

 rocks a quasi-fluid condition, and hence we cannot say that flexing attests 

 to a slow rate. On the other hand, faulting, without considering other con- 



