668 EBUPTIVE BOCKS FROM MONTANA— MEBRILL. vol.xvii, 



refusion, such as almost completely destroyed origiual structures with- 

 out wholly obliterating the minerals belonging to the first stages of 

 consolidation. The mere rounding alone of either mineral could not 

 be considered as indicalive of other conditions than are so frequently 

 shown by those minerals which belong to the earliest generation and 

 which, owing to reelevation of temperature or diminution of pressure 

 have become again partially dissolved by the molten magma. Among 

 crystals which develop freely in a more or less viscid magma, however, 

 no such interference as here shown could have occurred, and we must 

 conclude that their first crystallization took place under more restricted 

 circumstances. 



The gray material constituting the greater portion of the ground- 

 mass is here not sufficiently crystalline for optical determination.^ 

 Under a i)0wer of 170 diameters it shows only a scaly, granular aggre- 

 gate of a colorless mineral or minerals, polarizing in light and dark 

 colors, with the individual granules blending into one another as the 

 stage is revolved; often an imi)erfect, spherulitic structure is devel- 

 oped. The ap])earance suggested that nepheline or meUlite might l>e 

 one of the constituents, but inicro-chemical tests failed to show a trace 

 of either mineral, though an analysis of the ])ortion soluble in hydro- 

 chloric acid (p. (570) is very suggestive. From the high percentage of 

 silica and ])otash shown by the complete analysis it must be inferred 

 that an acid feldspar is a i)rominent constituent. Occasional areas of 

 colorless glass are seen, but by far the greater part of the gronndmass 

 is composed of the white substance, presenting always the peculiar 

 scaly-granular structure above described, and which is unlike anything 

 I now recall, excei)ting as sometimes displayed in rocks of the phonolite 

 or trachyte groups, Very evenly distributed throughout the entire 

 gronndmass are innumerable small flecks of brown mica and augite 

 microlites. These last are peculiarly beautiful and interesting, showing 

 every stage from mere skeleton outlines inclosing areas of groundmass, 

 elongated, needle-like forms with crenate and undulating borders, to 

 quite i)erfectly outlined crystals. 



As shown in the section they are faintly greenish, or nearly colorless. 

 Between crossed nicolsthe larger forms show cores giving lively bluish 

 or purple polarizations colors, while the borders are very faint yellow. 



Although small and imperfect, the optical and crystallographic 

 properties are readily determined, and agree with those of normal 

 augite. No microlites were observed which could with certainty be 

 referred to olivine. The abundant small scales of brown mica are 

 scattered singly and in small clusters quite uniformly throughout the 

 groundmass. It is noticeable, however, that in the immediate vicinity 

 of the corroded augites they often occur in greater abundance, and in 

 particular where augites and olivines lie in close juxtaposition. The 

 space is then often filled with a perfect cloud of the small mica scales, 

 as I have attempted to show in figure 10. I think that there can be no 

 doubt that these, and perhaps all the micas, and augite microlites as 



