OGOUERENCE AND SUCCESSION OF ROCKS. 193 



tion of Gold Hill. To the north of the Union shaft the porphyritic diorites 

 swing to the northeast. On the surface they disappear under the andesites, 

 while underground the explorations north of the Opliir have been almost 

 wholly confined to the dioritic area, and afford no means of tracing the 

 extension, of the diorites beneath the cap. Near where the contact between 

 the diorites and the diabase probably occurs are the heavy croppings known 

 as the Scorpion. Whether these actually correspond to the contact or not 

 can only be told by exploration ; but, if not, that contact has left no trace 

 upon the surface in this region, which would be very remarkable if the 

 deductions made in the last chapter as to the' age of the Lode are correct. 

 It is not unlikely that the dioritic rocks are continuous, or nearly so, under 

 the Flowery Ridge, and are thus connected with the occurrences at and 

 near the Lady Bryan. Diorite seems to have preceded the quartz-porphyry, 

 for it occurs in the Justice, and in the Caledonia, beneath the porphyry. 



Relations of porphyritic to granitoid forms. — Thc rclatious of the dioHtic porphyries 

 to the granular mass are interesting. The former are constantly found over- 

 lyin<>- the granular rock, but a line of demarkation can seldom be drawn, 

 transitions and mixed masses being of constant occurrence. Roughly the 

 area between Bullion and Spanish ravines is granitoid, and the masses 

 beyond these limits porphyritic; but this is a very rude approximation, for 

 fine porphyries occur in the very midst of the mass of Mount Davidson, 

 and granular patches are to be found throughout the hornblendic porphyries. 

 The micaceous porphyries also appear to overlie the hornblendic variety, 

 into which, however, they merge. The conditions suggest a physical explana- 

 tion Some geologists now believe that the crystalline structure of rocks 

 depends solely on the pressure under which they have consolidated. Such an 

 explanation of the present case, however, seems to me unsatisfactory. The 

 variation in a horizontal direction is nearly as marked as that in a vertical 

 line, and though there is an exposure of at least 2,500 feet, vertically, allow- 

 ino- for the displacement by faulting, the deepest granular diorites are not 

 more coarsely crystalline than those on the top of Mount Davidson. Nor 

 are the other rocks from the bottom of the mines in any perceptible manner 

 different from those collected at or near the surface. The cause of the differ- 

 ence between the granular and the porphyritic diorite, if these rocks are ad- 

 13 c L 



