5i C. R. Longivell — Geology of 



greater thickness. Apparently the maximum exposed 

 thickness of the sediments is nearly 2,000 feet, and an mi- 

 determined amount has been removed by erosion. The 

 formation has the essential characteristics of interior 

 basin deposits in an arid or semi-arid climate. No diag- 

 nostic fossils were found in these beds, and reference of 

 the formation to the Pliocene is made tentatively, because 

 of apparent correspondence to deposits of that age near 

 Panaca, 100 miles to the north. 



Quaternary deposits. — Various alluvial deposits are 

 distinctly younger than the sediments of probable Plio- 

 cene age, and will be grouped as Quaternary, with the fol- 

 lowing classification according to their nature and origin : 

 (1) Coarse gravels and sand, with a thickness ranging 

 from 10 to 300 feet, underlying high surfaces of aggrada- 

 tion. These deposits are practically uncemented except 

 in the upper 4 or 5 feet, where a rich matrix of calcium 

 carbonate or gypsum forms a resistant caliche. (2) Rem- 

 nants of consolidated fan, talus, and travertine deposits 

 adjacent to mountain walls. (3) Alluvial fans still or 

 until quite recently in process of building. (4) Recent 

 talus cones and slope covers. (5) Valley filling of 

 banded silt and sand of undetermined thickness. (6) 

 Sand dunes, in river valleys and on terrace surfaces. 



Igneous rochs. — The north end of the Black Mountains 

 is composed mainly of intrusive porphyries and buried 

 flows, all probably of Pliocene age or later. A thin flow 

 of augite-olivine basalt, apparently derived from the 

 Black Mountain volcanic center, is included in the saline 

 clays of Virgin Valley and extends northward almost to 

 St. Thomas. In Grrand Wash Valley two flows of oli\ine 

 basalt, separated by a vertical interval of 70 feet, are 

 buried in Quaternary gravels. Apparently their source 

 was to the north. 



Structure. 



The region has been affected by both faulting and fold- 

 ing, and the structure is complicated. Four principal 

 groups of structural features are recognized, as follows : 

 (1) Great faults or fault zones which separate adjacent 

 ranges. (2) Intersecting faults which divide each range 

 into a mosaic of blocks. (3) Folding, both moderate and 

 intense. (4) Overthrusting, apparently of a major char- 

 acter. The intermontane faults will be discussed first. 



