198 GEOLOGY OF THE OOMSTOCK LODE. 



encountered. On this point, however, there is some uncertainty, for before 

 the identification of diabase in the east country much of the hanging wall now 

 exposed would undoubtedly have been recognized as an older rock and 

 confounded with diorite. The stringers of diabase in the Sierra Nevada and 

 the Utah mark fissures unquestionably belonging to the Comstock system, 

 and that in the former mine at least Avere accompanied by a very trifling 

 amount of ore. The history of the Lode and the chemical discussions 

 which form the subjects of other chapters, make it highly improbable 

 that bodies of any consequence will ever be found near these stringers. 

 The main contact of the diabase with the diorites swings sharply to the 

 northeast in the Sierra Nevada ground, and has not been explored beyond 

 that point. Diabase does not appear south of the Overman, and the Forman 

 shaft passed from hornblende-andesite into quartz-porphyry at 2,200 feet 

 from the surface. If, therefore, as there is reason to believe, the latter rock 

 preceded the diabase, this will not be encountered in the Forman shaft. The 

 extension of the diabase in an easterly direction is somewhat uncertain. On 

 the line of the Sutro Tunnel the diabase is only about l,oOO feet wide, 

 measured horizontally. It is certainly wider than this at the Osbisfon shaft 

 and the new Yellotv Jacket. . The Osbiston is believed to have met diabase at 

 a depth of about 1,000 feet, though the locality was not accessible, while 

 the new Yellow Jacket passed iuto it at less than 400 feet from the surface, 

 indicating an extensive body still farther east. 



The lithological varieties of the diabase have been sufficiently described 

 in a former chapter. In structure it resembles the diorite, being split up 

 near the Lodk into rough sheets parallel to the main fissure, as has been 

 explained in Chapter IV. I have been wholly unable to see any evidence 

 that this rock was not emitted at a single outbreak. Its position, lying as a 

 mass upon a diorite wall sloping at an angle of about 45°, together with 

 the details of the relations of the two rocks, shows that it is younger than 

 the diorites. That it is also probably younger than the quartz-porphyry 

 is shown by the occurrences in the Overman, which are not fully satisfactory 

 only because they are so limited. 



"Black dike. — The younger diabase, as has been seen, is identical with 

 the trap of New Jersey. It has often been confounded with the black slates 



