ROCK FORMATIONS ON IRON HILL. 383 



much finer grain, and its minute feldspar crystals are generally of a flesh 

 color. When thoroughly bleached by decomposition it can be distin- 

 guished from the White Porphyry by its speckled or mottled appearance, 

 whence the name of "mottled porphyry" that is not infrequently applied 

 to it. It is probably also a variety of Gray Porphyry, though, like the pre- 

 ceding, not found in sufficiently fresh condition for exact determination. 



As nearly as can be determined from the various prospect holes on the 

 slope of the hill, this body has its maximum thickness near the line of the 

 Iron fault and thins out to the southeast. It is best seen in a tunnel driven 

 in near the fault, on its contact with an underlying limestone, which is sup- 

 posed to be the lower portion of the Blue Limestone, though, as the Part- 

 ing Quartzite was not actually exposed below it, this cannot be regarded as 

 beyond a doubt. A certain amount of iron-stained material is found at the 

 contact, and it had been supposed by some that this repetition of a contact 

 of porphyry and undei'lying limestone below the regular outcrop was evi- 

 dence of another fault, the different chai-acter of the two porphyries having 

 escaped observation. 



This porphyry sheet is probably of much wider extent than the one 

 previously described, although its actual outcrop is much more limited ; as 

 will be seen later, it probably extends under the greater part of Carbonate 

 Hill, and inasmuch as sheets of Gray Porphyry are found in considerable 

 development on the north end of Iron Hill, though at somewhat lower hori- 

 zon, it is fair to assume, as has been done in the sections (Atlas Sheet XXIV), 

 that it extends under Iron Hill also, gradually lowering in horizon toward 

 the north. It is probable that the small bodies of Gray Porphyry found 

 crossing the limestone in various points of the hill are offshoots from this 

 body. 



White Porphyry. — The White Porphvry, which forms the summit of the 

 hill, is the normal rock already described. From the quarry in California 

 gulch, just above Graham gulch, was taken the specimen chosen for complete 

 analysis (see Appendix B, Table I). In this quarry, which is but a short 

 distance west of the Iron fault, the jointing planes are strongly marked, 

 those parallel with the plane of the fault being the most prominent. 



