Vol. 4] 



Lawson. — The Robinson Mining District. 



289 



Sierra Nevada, the Humboldt Mountains,* the Wasatch, f Cen- 

 tral Idaho, $ and the Coast of British Columbia. § 



At a later date, probably in Tertiary time, there were local 

 intrusions of a light colored, acid, porphyritic rock which has 

 since been much kaolinized and silicified, in which or in con- 

 nection with which important ore bodies are now found. While 

 there is thus a certain degree of complexity of geological struc- 

 ture within the range, the structural lines seem to have had little 

 or no influence in determining either the east or the west border 

 of the range, both of which are remarkably linear, and would 

 appear, from a very partial and cursory examination, to cut 

 obliquely across various structures indifferently, a fact which 

 in itself justifies the conception of the range as an orographic 

 block of the Basin Range type. This independence of the bor- 

 dering lines of the range to the structure is in marked contrast 

 with the control which has been exercised by the structure upon 

 the geomorphic development within the range. Here the geo- 

 morphic expression is conditioned by the structure, and by the 

 differential resistance of the rocks to erosion. In the softer 

 formations the valleys are broad and the slopes are quite mature. 

 In the harder formations the streams run in canons. In general 

 the montaine geomorphy is much more advanced than in the 

 case of the ranges of the same structural type recently described 

 by Louderback|| in the western part of Nevada, a contrast which 

 may be due, perhaps wholly and certainly in part, to the absence 

 of a cap of basaltic lava in the Egan Range. After the geo- 

 morphy of the range had been well advanced toward its present 

 condition there were local volcanic eruptions which caused some 

 of the valleys to be occupied to a limited extent with tuffs and 

 flows of lava. Since then the range has been faulted and a con- 

 siderable proportion of these lavas and tuffs has been removed 

 by erosion, and the valleys have been deepened below the floors 

 upon which the volcanic rocks rest. 



The Robinson Mining district, with the geology of which this 



* Louderback, G. D. Basin Eange structure of the Humboldt Beeion 

 Bull. G. S. A., Vol. 15, pp. 289-346, 1904. 



t Emmons, S. F. Am. Jour. Sci., 4th ser. Vol. XVI, pp. 139-147 1903 

 i Lindgren, W. 20th Ann. Kpt. U. S. G. S., Pt. Ill, pp. 65-256.' 

 § Dawson, G. M. Geol. Surv. Canada Ann. Kpt., 1886, Pt B 

 II Op. ext. 



