24 



THREE GEOLOGICAL PROVINCES. 



In explanation of the diagram Mr. Gilbert remarks: 

 "The sections accumulated by our geological observers admit of the 

 following classifications: 



A B C D E F 



"1. Faulted nionoclinals occur, in which the strata on one side of the 

 fault have been lifted, while those on the opposite side either do not appear 

 A), or (less frequently) have been elevated a less amount (B). Two-thirds 

 of the mountain ridges can be referred to this class. 



"2. Other ridges are uplifts limited by parallel faults (C), and to these 

 may be assigned a few instances of isolated synclinals (D), occurring under 

 circumstances that preclude the idea that they are remnants omitted by 

 denudation. 



"3. True anticlinals (E) are very rare, except as local, subsidiary fea- 

 tures, but many ranges are built of faulted and dislocated rock masses (F), 

 with an imperfect anticlinal arrangement. 



"Not only is it impossible to formulate these features, by the aid of any 

 hypothetical denudation, in such a system of undulations and foldings as the 

 Messrs. Rogers have so thoroughly demonstrated in Pennsylvania and Vir- 

 ginia, but the structure of the Basin Range system stands in strong contrast 

 to that of the Appalachians. In the latter, corrugation has been produced 

 commonly by folding, exceptionally by faulting; in the former, commonly 

 by faulting, exception " '-Hy by flexure. In. the latter, few eruptive rocks occur; 

 in the former volcanic phenomena abound, and are intimately associated with 

 ridges of upheaval. The regular alternations of curved anticlinals and syn- 

 clinals of the Appalachians demand the assumption of great horizontal dimi- 

 nution of the space covered by the disturbed strata, and suggest lateral 

 pressure as the immediate force concerned; while in the Basin Ranges, the 

 displacement of comparatively rigid bodies of strata by vertical or nearly 

 vertical faults involves little horizontal diminution, and suggests the appli- 

 cation of vertical pressure from below." 



Thus a characteristic range of this country is the edge of a great block 



