312 University of California Publications. [Geology 



and a general absence of ferromagnesian minerals. Its more 

 precise characterization and its variations will appear from the 

 detailed descriptions to be given after its broader features as a 

 factor in the geology of this field have been considered. 



Extent. — An inspection of the geological map of the district 

 will show that the porphyry occupies two quite distinct areas. 

 One of these extends from Copper Flat westward to Rusty 

 Ridge at the west of the map. This area has an east and west 

 extent of nearly three miles and a maximum width of five-sixths 

 of a mile. The area is continuous and there are no outlying 

 patches of the same rock in its vicinity. 



The second area extends from Ocher Valley eastward to a 

 little beyond the eastern limit of the map near the Chainman 

 Mine. This area has also an east and west extent of about three 

 miles and a maximum width at the Ruth Mine of about three- 

 fifths of a mile. This area is more irregular in outline and less 

 continuous than the western area. It has in fact, in consequence 

 of its dissection by erosion, been separated into two distinct 

 parts, the Jess extensive of which is in the vicinity of the Saxton 

 and Chainman Mines. These two areas of porphyry lie in a 

 distinct east and west allignment and form a belt of rock tra- 

 versing the district which has determined not only the distribu- 

 tion of the economically important minerals but also the evo- 

 lution, under erosion, of the geomorphic features of the region. 

 It is noteworthy that this allignment and the features deter- 

 mined by it are transverse to the general trend of the Egan 

 Range and to its dominant structural lines. 



The break in the continuity of the porphyry belt at Ocher 

 Valley in the central part of the district might on a rapid survey 

 be referred to its dissection by erosion, but a more careful ex- 

 amination of the contact between the porphyry and the adjacent 

 rock shows that there is little warrant for this view, and that 

 the two areas probably have always been distinct at the surface, 

 although doubtless connected at depth. 



Intrusive Relations. — The evidence as to the relations be- 

 tween the porphyry and the adjacent Palaeozoic formations is 

 in general quite satisfactory and establishes the fact that the 

 porphyry is intrusive in the sedimentary rocks. This evidence 



