110 CALIFORNIA ACADEMY OF SCIENCES. 



assert that my lines of demarcation are faultless. I can 

 only say that they were laid down with the most scrupulous 

 <;are and as the result of arduous labor, and that I know of 

 no errors. If, however, it may hereafter prove that I have 

 erroneously determined some slide, specimen or cropping, 

 here or elsewhere in the district, this will not invalidate the 

 general truth of my conclusions. 



The case of the micaceous diorite is precisely analagous 

 to that of the porphyritic hornblende diorite. Mica occurs 

 in patches on the bare rock surfaces of Mount Davidson — 

 here a flake or two, there a group fading out into rock in 

 which there is no mica discoverable. These occurrences 

 are less striking than those of porphyj^itic diorite in the 

 region immediately above the central group of mines, be- 

 cause the presence of mica at this point is unattended by 

 any physical or structural modification of the granitoid 

 mass. To the north of Spanish Eavine there is an increase 

 in porphyritic forms, both micaceous and hornblendic, but 

 the change is very gradual, and as typical granitoid diorite 

 occurs here as on Mount Davidson. If the micaceous rock 

 is all later hornblende andesite, as Messrs. Hague and 

 Iddings pronounce it, then Mount Davidson is later horn- 

 blende andesite. 



^^ White 7vck/' — Messrs. Hague and Iddings assert that 

 some white rocks found in the tunnel are identical with the 

 rock called, in my report, felsitic quartz porphyry. The 

 white rock contains no original quartz, but abundant sec- 

 ondary grains. It is connected macroscopically and micro- 

 scopically by transition with less altered andesites. This 

 can be shown from some of the slides referred to by Messrs. 

 Hague and Iddings as the white rock, when compared with 

 others which they recognize as andesites. An exactly simi- 

 lar case is exposed on a very large scale by the cuttings 

 made in the hillside to gain space for the Combination 

 Hoisting Works. Heie typical hornblende andesite is in- 

 tersected by a belt of solfataric action; and every imagin- 



