SACRAMENTO PORPHYRY. 81 



the Printer Boy and Josephine Porphyries, which occur the one on Printer 

 Boy, the other on Long and Derry hill. Among rocks so thoroughly 

 decomposed as are those in the immediate vicinity of the ore bodies it is 

 often impossible to assign an occurrence with absolute certainty to a dis- 

 tinct type; the miner can, however, in most cases distinguish these porphyries 

 from the White Porphyry by the outlines of former crystals which the slight 

 stain of iron oxide caused by their decomposition leaves. 



SACRAMENTO PORPHYRY. 



This rock in the hand specimen has the same general appearance as 

 the variety of Lincoln Porphyry which has no large crystals. It is a dark- 

 gray, granular, rather even-grained rock, in which the groundmass is decid- 

 edly subordinate, and contains quartz, two feldspars, biotite, and horn- 

 blende. It is distinguished from the former rock by carrying a much larger 

 proportion of plagioclase feldspar, and hornblende as well as biotite. The 

 microscope discloses the usual accessory minerals, with allanite and pyrite, 

 and shows that the groundmass is holocrystalline and contains no glassy 

 material. In the large masses of the higher mountain region it is usually a 

 fresh-looking rock, but in mine workings and under a covering of soil and 

 gravel capable of holding water it is usually much decomposed and bleached 

 to a light-green, almost homogeneous-looking rock, with much epidote. The 

 processes of decomposition in this rock, which are exceptionally interesting, 

 are explained at length in Appendix A. 



Occurrence. The main laccolitic body of Sacramento Porphyry is found 

 under Gemini Peaks, between the heads of Big and Little Sacramento 

 gulches. A fine cliff section of the* body is also found on the face of Mount 

 Evans towards Evans Amphitheater. It reaches a thickness of over a 

 thousand feet in this i*egion. Its main sheet occurs above the White Por- 

 phyry, or, when this is wanting, with an interposition of Weber Shales 

 between it and the Blue Limestone. East of the London fault it rests 

 directly on the Blue Limestone, and in the neighborhood of the Sacramento 

 mine it plays the same role with regard to the ore deposits that the White 

 and Lincoln porphyries do at other points. It also forms sheets higher up 

 in the Weber Grits and less frequently in the lower Paleozoic strata In 



MON XII 6 



