Vol. 4] 



Lawson 



. — The Robinson Mining District. 



331 



ing line between the oxidized and the unoxidized rock and their 

 absence above that line leave no room for doubt but that these 

 minerals formerly existed in the upper zone, and have been oxi- 

 dized and leached out by meteoric waters descending from the 

 surface. 



While there is thus a very sharp line between the two condi- 

 tions of the porphyry in the mine, that line is hypsometrically 

 extremely irregular, and has no reference to the present level 

 of the ground water. The water level stands in the mine at 

 present at about 335 feet below the shaft mouth, and the mean 

 lower limit of oxidation is somewhere between 100 and 150 feet 

 below the shaft mouth. But the various levels and cross cuts 

 of the mine have shown that tongues or wedges of oxidized rock 

 extend irregularly down into the unoxidized zone as far as the 

 500-foot level, with occasional limonitic stains along seams to 

 still greater depths. It appears to be clear from this state of 

 affairs that the level of the ground water has been lowered prob- 

 ably not less than 250 feet and possibly much more in quite late 

 geological time at such a rate that the oxidation process failed 

 to keep pace with it ; so that about a couple of hundred feet 

 of the unoxidized zone has been left stranded, so to speak, above 

 the water plane. The irregular tongues and seams of oxidized 

 rock that project down into the sulphide zone, of course, merely 

 represent the loci of more efficient oxidation by waters perco- 

 lating down from the surface, in the general tendency of that 

 process to overtake the retreating ground water. 



This lowering of the ground water may be explained in more 

 than one way. As will be shown later, the region has suffered 

 deformation by faulting and tilting at quite a late date in its 

 geological history and this deformation would, or at least might 

 easily, have affected the position of the water plane. Again the 

 district lies in a geological province which, as has been so well 

 shown by Gilbert* and Russell, f has been affected in Quater- 

 nary time by an advent of arid conditions extending to the pres- 

 ent, which may well be considered as an adequate cause of the 

 lowering of the ground water throughout the Great Basin. In 



* Lake Bonneville, U. S. G. S., Mon. I. 

 t Lake Lahontan, U. S. G. S., Mon. IX. 



