78 GEOLOGY OF THE YELLOWSTONE NATIONAL PAKK. 



They are much less abundant and smaller in most of the present group, 

 and are in part cracked and altered in the same manner as in the hornblende- 

 mica rocks. In some instances the feldspar is perfectly fresh when the 

 hornblende is entirely altered, and vice versa. The only other constituent 

 that may be classed with the phenocrysts is magnetite, in small crystals, 

 scattered through the rock, and inclosed in the hornblende. It is in some 

 cases decomposed and replaced by aggregations of hydrous oxide of iron, 

 leucoxene, and brightly polarized scales of an undetermined mineral. The 

 magnetite is plainly titaniferous, and chemical analysis shows 1.71 per cent 

 of titanium oxide in the rock analyzed Pyrite is occasionally present. 



The groundmass, always holocrystalline, consists in most cases of a 

 microcrystalline aggregation of grains that are indistinctly poikilitic, and 

 of feldspar microlites, both lath-shaped and rectangular, together with 

 magnetite and irregularly shaped hornblende or chlorite. In one case the 

 micropoikilitic structure is more pronounced. In others the lath-shaped 

 and rectangular feldspars preponderate. In two instances the groundmass 

 is microcryptocrystalline, with scattered feldspar microlites and some indis- 

 tinct micropoikilitic structure. 



On the ridge running southwest from Electric Peak three distinct 

 bodies of intrusive rock intersect one another. The oldest is a somewhat 

 altered hornblende-andesite-porphyry (185). This is traversed by a large 

 sheet of hornblende-andesite-porphyry, which is quite fresh, light gray 

 colored, with abundant small phenocrysts of hornblende and feldspar. 

 Through the rock are segregations, several inches in diameter, that differ 

 greatly in texture and structure. Some appear as coarser-grained modifi- 

 cations of the main rock, others as varieties with larger hornblende. Some 

 have a laminated or gneissoid structure. Occasionally small dark patches 

 surround red garnets (182, 184, 188, 189). The main body of the rock, 

 whose chemical composition is given in analysis 1, p. 81, consists of a 

 micropoikilitic groundmass, with abundant lath-shaped and rectangular 

 feldspars, besides hornblende, a little chlorite, magnetite, and quartz, carry- 

 ing phenocrysts of hornblende and cracked plagioclase. It is remarkable 

 that by the side of greatly cracked crystals of plagioclase, with the cracks 

 filled with the secondary mineral already described, the crystals of horn- 

 blende exhibit no cracking, often no cleavage, and no optical strain 

 phenomena. The question arises whether or no the cracking of the feld- 



