102 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



similar intergrowths of hornblende and pyroxene in unaltered, often pumi- 

 ceous, glassy lavas. They are to he distinguished from the formation of 

 hornblende from pyroxene by secondary processes of metamorphism, 

 which may also lead to the production of compact green hornblende. The 

 transformation of both pyroxene and compact hornblende into fibrous 

 amphibole has taken place in some of these rocks to a limited extent. They 

 are in general very fresh and unaltered. 



II b. VARIETIES IN WHICH THE LIGHT-COLORED MINERALS ARE IN EXCESS, BUT 

 IN WHICH QUARTZ IS NOT EXCESSIVE. 



These include the more feldspathic facies of the diorite, whether 

 within the main body, on its margin, or as dikes or veins within the mass. 

 There is considerable quartz and a range of ferromagnesian minerals that 

 connect them with the preceding varieties. The finer-grained forms approach 

 the dike rocks in microscopical characters, and are possibly connected with 

 them geologically. 



Hornblende, pyroxene, and biotite are similar to the same minerals in 

 the main body of diorite, and exhibit the same relations to one another 

 when in juxtaposition, but vary more widely in their relative proportions, 

 as shown in Tables V and VI. The feldspars are somewhat more alkaline, 

 and have a slightly different habit from those in the main diorite. The 

 quartz plays a somewhat different role. Its crystals occasionally possess 

 a crudely idiomorphic form, and because of their greater abundance the 

 microstructure exhibits more of a granitic appearance than in the less 

 siliceous diorites. 



The coarsest-grained variety (313) consists of broad plagioclase feld- 

 spars from 1 mm. to 2 mm. long, with numerous small quartz grains located 

 along the line of junction of the feldspars; hornblende and biotite occur in 

 irregularly shaped anhedrons, the hornblende being in part phenocrystic. 

 Magnetite and apatite are also present. When the grain of the rock 

 becomes smaller the feldspars stand out more prominently in a fine-grained 

 groundmass. The porphyritic texture is quite pronounced in specimen 

 308 from near contact with sedimentary rocks. Its grade of crystalliza- 

 tion is 27. The large feldspars and hornblendes are idiomorphic. The 

 crystals composing the groundmass exhibit an approach to idiomorphism, 

 especially the feldspars, and to some extent the quartzes. This structure is 

 shown in fig. 1, PL XX. 



