382 GEOLOGY AND MIOTNG INDUSTRY OF LEADVILLE. 



Later intrusive sheets. — Besides tliis normal series of beds, are two inti'usive 

 sheets of porphyry of later eruption than the "White, and allied to, though 

 not absolutely identical with, the Gray Porphyry. One of these is found 

 at the top of the Blue Limestone, the other near its base. Their probable 

 extent can be best seen by reference to the map and sections (Atlas Sheets 

 XXIII, XXIV, XXV). The thicknesses there given are assumed from the 

 position of outcrops, where they could be determined, and from other in- 

 direct evidence, and may differ considerably from the actual facts, as these 

 porphyry sheets, especially the later ones, vary much in thickness in rela- 

 tively short distances. 



Upper sheet. — The rock of the former of these bodies is of a dark- gray 

 color and consists of plates of altered mica and relatively large, opaque, 

 white feldspars in a greenish-gray matrix. So far as seen it is in a too 

 advanced state of decomposition to allow of a satisfactory determination of 

 its original constituents. Externally, however, it resembles more closely 

 the country rock of the Printer Boy mine than any other i^orphyrj^ col- 

 lected. 



This sheet, while in general separating the White Porphyry fi-om the 

 Blue Limestone, does not always keep exactly the same horizon. In the 

 bed of California Gulch, where the outcrops cross and where this porphyry 

 seems to be thickest, it cuts into the Blue Limestone, leaving a portion of 

 the latter above it, near the mouth of the La Plata tunnel. Farther west, 

 on the hill slopes, it cuts up into the White Porphyry for a short distance, 

 leaving a sheet of that rock between it and the Blue Limestone, and then 

 again returns to the contact on the Lime claim, on Iron Hill, and west of the 

 Dome fault, on Dome Hill. There is direct evidence that the sheet thins or 

 wedges out from this crossing of California Gulch to the south, .west, and 

 north, but on the east no workings have yet reached a sufficient depth to 

 cut it. It is not impossible that it may be an offshoot from some large body 

 occupying a lower position in that direction — the Printer Boy body, for 

 instance, which is at a lower geological horizon, though actuallj' brought to 

 a higher elevation by faulting. 



Lower sheet. — The rock of the second body, as compared with that just 

 descril^ed or with the normal Gray Porphyry, has in the hand specimen a 



