24 MmiNa IFDUSTEY. 



Comstock lode is, in fact, this downward projection of the syenite slope, and 

 its contours agree very closely with those traced upon the surface. It 

 seems probable that in the Post-Tertiary erosion period this syenite nucleus 

 of the district was very little modified ; its already sculptured ravines would 

 have received the modern drainage and poured it out upon the later eruptive 

 rocks at the mouths of its own canons ; so that the fact of the present water- 

 courses continuing alike across the syenite and propylite is no evidence that 

 they were formed at the same time. A brief study of the contour lines of 

 Mount Davidson and of the geological cross-sections of the Comstock lode 

 will show that the slope angle has almost no change beneath the propylite, and 

 the recess which takes place in the Savage and the Gould and Curry will be 

 found to almost exactly coincide with the broad recurve which scoops the 

 whole front of Mount Davidson ; so, also, the spur which makes out from the 

 southeast foot of the mountain is carried downward through the Chollar 

 Potosi and is there a barrier to the deposition of ore. 



Syenite of a similar character, and bearing the same relation to the ancient 

 series of metamorphic rocks, is found in different localities over the whole basin; 

 , its mode of occurrence is always the same. Wherever observed it always accom- 

 panies the mountain fractures of the Jurassic period, and is clearly distinct 

 from the syenitic granite, which is also a prominent feature of this period; it 

 seems to form a connecting link, both in age and geological features, between 

 the granitic family and the more modern volcanic group ; always later than the 

 granite and always earlier than the propylite, it is more closely connected with 

 the former than the latter, both as regards its lithological habit and in the 

 interior structure. 



It shows no tendency here or elsewhere in the Grreat Basin to pass into 

 the syenitic granite. Ordinarily of a finer texture and more compact structure, 

 it is clearly to be distinguished from that rock — the titanic iron of the one is 

 never found in the other; nor does the quartz, which is a constant accompani- 

 ment of the syenitic granite, ever appear in the more basic syenite, yet their 

 outward resemblance is very great, and they are often mistaken one for the 

 other. 



The transitions described between the two by Von Cotta are not observed 

 here. Of its connection with the folded rocks, it can only be said, that it pene- 



