64: GEOLOGY OF THE GOMSTOCK LODE. 



borders; but in one very glassy rock it lacks this accompaniment. The 

 feldspars are also entirely similar to those in the preceding rock. Anorthite 

 has been identitied in a few slides among the larger crystals, but in most 

 cases the maximum angles of extinction correspond to labradorite. The 

 niicrolitic feldspars appear generally to be oligoclase. The iron ore is com- 

 monh- magnetite, but in a few cases characteristic ilmenite sections have 

 been observed. Apatite is invariabl}' present, very frequently as brown or 

 dusty crystals. There is no inconsistency between the presence of the brown 

 apatite and the colorless variety, which often occur in profusion in the same 

 slide; but the brown crystals seem rarely to assume the acicular form which 

 so generally prevails among colorless apatites. I have not observed a single 

 zircon, nor anything which can be set down with certaint}- as titanite. The 

 gronndmass of tlie augite-andesites is usually microlitic, though in one or 

 two cases granular structure has been noted. It is very frequently the 

 case that the microlites of feldspar are excessively minute, and with lower 

 objectives the groundmass then gives the impression of felt. This is an 

 appearance which the hornblende-andesites seldom present. The microlites 

 are often so arranged as to produce the effect called fluidal structure. 



Field character. — Tlic Ordinary variety of augite-andesite in a fresh condition 

 is dark blue, or brownish-blue, in color, resinous in luster, and has a rough 

 fracture. The comparatively fine-grained varieties often show the lighter 

 colors and smoother fractures common in hornblende-andesites, and when 

 the rock is at the same time somewliat hornblendic it is readily confounded 

 with hornblende-andesite. Sometimes, when the feldsi)ars are unusually 

 developed and the fracture is excessively rough, the rock might be mistaken 

 for trachyte ; but the absence of mica, the rarity of the hornblendes, and the 

 predominance of triclinic feldspars are generally sufficient to distinguish 

 it. In a few instances the augite-andesites are very granular and coarse- 

 grained, and when slightly decom2:)osed do not greatly differ from some dio- 

 rites in appearance, but the likeness is superficial. An imperfect columnar 

 structure is occasionally met with, but is not characteristic of the rock. 

 Breccia is exceedingly common, and is sometimes tufaceous. 



Decomposition and weathering. — As Is thc case witli thc hombleude-andesites, 

 when the rock is directly exposed to the action of the atmosphere the process 



