360 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



sist chiefly of one iudividual, iu which a few thiu wedges are inserted at one end, or- 

 ou one side, in twinning position. These are, I presume, the crystals described by 

 Captain Dutton as orthoclase at one end and plagioclase at the other. A zonal strnct- 

 ure is often present, whicli is at times interrupted by the twinning. Inclusious in 

 the plagioclase are not very abundant. There are sometimes minute dark inclusions, 

 regular or ii-regular in shape and arrangement, which seem to be early inclusions of 

 the groundmass or devitrifled matter. Distinct glass and fluid inclusions were seldom 

 found. Hornblende occasionally penetrates the feldspar, but inclusions of other min- 

 erals are rare. 



The hornblende itself is well developed crystallographically. In this particular 

 case (04) it shows an unusual tendency to an alteration, by which dark ore grains are 

 formed on and adjoining the outer surface and on cleavage and other fissure planes. 

 The appearance is, however, entirely difterent from that of andesitic hornblende. In 

 one place hornblende is apparently forming from pale pyroxene. This is, however, 

 an isolated case, as elsewhere the distinct outlines of the hornblende crystals j^rove 

 them to be original as such. The hornblende is green and fibrous rather than com- 

 pact and yellow. 



Titanite, which appears in most of these rocks, does not seem to be present here 

 in good crystals. Apatite is not abundant, but occurs in short, stout prisms. But little 

 magnetite occurs in large grains. 



The gyou7Hhna.ss is granular throughout and has the same composition as in the 

 Mosquito Eange porphyrites; that is, it consists chiefly of quartz and orthoclase. 



Of the other Henry Mountain rocks, Nos. 8, 9, 16, IS, 20, 23, 32, 35, 40, 44, 46, 47, 

 and 50 — thirteen iu all — seem identical in all essential points with that described 

 above. Other accessory minerals appear iu some of these sections. Biotite appears 

 as a subordinate constituent iu No. 35, corresponding in this respect to our porphyrites, 

 and being the sole case noticed. 



Isolated grains of pink garnet occur iu Nos. 23, 37, and 47. litanite is present 

 HI nearly all and ilmcuite in some of them. In 40 the latter seems to be producing 

 titanite through its alteration. In 46 and 23 (new section) I find allanite correspond- 

 ing exactly iu appearance to that of the Mosquito rocks. 



It cau hardly be asserted that plagioclase predominates iu all of the rocks, from 

 the evidence of these sections alone, as some of them are very small ; there can be no 

 doubt, however, that all belong to the same rock type. I cannot convince myself that 

 orthoclase exists in more than isolated crystals among the macroscopic elements. 



Inclusions in feldspar are seldom more numerous or distinct than iu the first case 

 described. Occasionally, however, a crystal is filled with minute hornblende micro- 

 lites and clear crystals of zircon, with other ill-defined matter. 



The feldspars are usually quite fresh, but the hornblende is sometimes entirely 

 decomposed. The common result is a mixture of chlorite, filmy calcite, and opaque 

 particles. Epidote is often a further product. Granular calcite is visible in some 

 cases and its origin doubtful. The minute ore grains of the groundmass are often 

 hydrated, giving a dingy tinge to the rock. 



In none of the above rocks can there be any question as to the thoroughly crys- 

 talline nature of the groundmass, but it varies in relation to the crystals and in com- 



