The diorite-porphyrite has a groundmass of idiomorphic hornblende, 

 lathshaped feldspars and some interstitial quartz, with phenocrysts of 

 the same minerals, but principally of feldspar. Among the diabases is 

 a quartzose variety. 



Eruptive Rocks from Montana. — Among some specimens of 

 eruptive rocks obtained from Gallatin, Jefferson and Madison Coun- 

 ties, Montana, Merrill 3 finds basalts, andesites, lamprophyres, syenites, 

 porphy rites, wehrlites, harzburgites and websterites, some of which pos- 

 sess peculiar characteristics. A hornblende andesite. for instance, con- 

 tains large corroded brickred pleochroic apatite crystals, whose color is 

 due to innumerable inclusions scattered through them. The ground- 

 mass of some of the basalts has a spherulitic structure. The wehrlite 

 is a holocrystalline aggregate of pale green diallage, reddish brown 

 biotite, colorless olivine and a few patches of plagioclase. Its structure 

 is estaclastic or gran uli tic, the larger crystals being surrounded by an 

 aggregate of smaller ones. The websterite consists of green diallage 

 and colorless eustatite with included foliae of mica and occasional inter- 

 stitial areas of feldspar, and is thus related to gabbro. Some of the 

 lamprophyres are composed of groups of polysomatic olivines or of oli- 

 vine and augite in a scaly granular groundmass of lighter colored min- 

 erals, through which are scattered^ small flakes of brown biotite and 

 tiny augite microlites. This structure is accounted for on the supposi- 

 tion that the granular groups of olivine and of olivine and augite belong 

 to an older series of crystalline products than those of the ground- 



Porphyrites and the Porphyritic Structure. — In a general 

 account of th< tins of Colorado, Utah and Arizona, 



Cross' gives a brief synopsis of the characteristics of the rocks that con- 

 stitute their cores. These rocks comprise augite, hornblende and horn- 

 blende mica-porphyrifes, diorites and quartz-porphyi ites. All contain 

 phenocrysts of plagioclase and of the iron bearing silicates, with the 

 feldspars largely predominating. These upon separating left for consol- 

 idation into the groundmass a magma which upon crystallization 

 yielded a granular aggregate consisting largely of quartz and ortho- 

 clase. No pressure effects were seen in any of the sections studied. All 

 are porphyritic with ft granular groundmass, which diflfers in the different 

 roeks, principally in the proportion of its constituents. The porphy- 

 ritic structure as defined by the author Is not the result of the recur- 



