362 GEOLOGY AND MINING INDUSTEY OF LEADVILLE. 



there is more or less of a gradation in size from the large ones to these in the ground- 

 mass. Very minute ore particles are present. 



No. 43 is of quite different macroscopical structure. It appears almost macrocrys- 

 talline, the grouudmass occupying simply the interstices between the small white 

 tablets of feldspar, while the augite occurs in minute grains not recognizable by the 

 naked eye. 



Microscopical The feldspars have a duller appearance even than those in 31, and 

 there is the same difficulty in determining which species predominates. The augite is 

 the same in character, but does not appear in the groundmass as in 31. 



In No. 28 (the slide alone examined) exists still another form of structure. The 

 whole mass is here microcrystalliue and consists chiefly of feldspar, concerning which 

 the same doubts exist as before. The augite is very distinct. The grouudmass is made 

 up of small feldspars and nearly every one is determinable as feldspar. Quartz does 

 not appear; the same accessory minerals are here as in others, titauitc, magnetite, &c. 

 Hornblende is exceedingly rare, if, indeed, it occurs at all in these three rocks. No. 

 29, however, shows both minerals. The hand specimen shows large, distinct horn- 

 blendes, but in the slide, among the few minute irregular grains (no large ones being 

 present), augite appears fully as abundantly as hornblende. The remainder of the 

 rock is entirely feldspathic, both orthoclase and plagioclase being recognizable. 



But one rock remains, No. 57. This is the sanidine-trachyte of Dutton. Not hav- 

 ing the hand specimen and with only one slide, but little can be made out of it. It 

 seems like a tufa or fragiuental rock of some kind. The minerals recognizable (plagio- 

 clase, orthoclase, quartz, and hornblende) are chiefly in irregular fragments of crystals 

 and the gronndmass, though cryptocrystalliue for the most part, has some isotropic 

 substance. 



BESUMlL 



. The greater part by far of the Henry Mountain rocks correspond very closely in 

 composition and structure to our Mosquito Eauge porphyrites, or in particular to those 

 varieties in which biotite is rare or is wanting and in which the hornblende does not 

 appear in the grouudmass in large quantity. Both consist of plagioclase and horn- 

 blende, with a granular grouudmass, composed essentially of quartz and orthoclase. 

 They differ 



a, in outward appearance. 



6, in almost total lack of biotite. 



c, in frequent presence of titanite. 



d, in that the grain of the groundmass sinks in certain cases to exceeding 



fineness. 



None of these is weighty in comparison with the resemblances. 

 The outward difference seems due to the fact that the specimens were taken from 

 the surface in a region essentially dry and arid. 



