ABSAROKITE AND SHOSHOiSTITE. 327 



The chief characteristics of the most basic class are the presence of 

 abundant phenocrysts of olivine and augite and the absence of any of feld- 

 spar. The groundmass may be anything from a dark glass to an almost 

 phanerocrystalline light-gray mass. It is oftener aphanitic and dark 

 greenish gray. The phenocrysts are large and pronounced in many cases,- 

 but are very small in others. The microscopical characteristics will be 

 given at length. Chemically they are low in silica, from 46 to 52 per cent; 

 low in alumina, from 9 to 12 per cent; high in magnesia, from 8 to 13 per 

 cent; comparatively high in alkalies, with potash considerably higher than 

 soda, except in one case. The molecular ratio of the alkalies to silica is 

 0.08 and 0.09. After the crystallization of abundant phenocrysts of olivine 

 and augite the remainder of the magma, owing to the low alumina and to 

 relatively high alkalies, was so constituted that alkali-feldspathic minerals, 

 in the form of orthoclase or leucite, might crystallize out, which they did 

 or not according to the conditions under which solidification took place. 



The principal characteristic of the second class is the presence of 

 phenocrysts of labradorite, together with those of augite and olivine, in a 

 groundmass that is usually dark greenish gray, with a semiwaxy luster, 

 but which may be glassy or phanerocrystalline, and which when distinctly 

 crystallized contains a notable percentage of orthoclase. They range from 

 varieties rich in olivine and augite, which, with decreasing labradorite, 

 grade into rocks of the first class, to varieties with few olivines or augites. 



Chemically they contain from 50 to 56 per cent of silica. Alumina is 

 moderate to high, from 17 to 19.7 per cent. Lime and magnesia are moderate 

 to low, the former from 8 to 4.3 per cent, the latter from 4.4 to 2.5. The 

 alkilies are moderately high, with potash comparatively high for rocks of 

 this region, with like amounts of silica, from 3.4 to 4.4 per cent, the soda 

 ranging from 3 to 3.9 per cent. The molecular ratios of the alkalies to 

 silica is 0.10 and 0.11. After the crystallization of phenocrysts of labradorite, 

 olivine, and augite, the remainder of the magma was rich in alkali-feld- 

 spathic material, usually that of orthoclase, which shows itself in the ground- 

 mass according to conditions of crystallization. 



Rocks corresponding chemically to both of these classes occur with no 

 megascopic phenocrysts, and in various phases of crystallization, from 

 glassy to holocrystalline, and almost phanerocrystalline; consequently they 

 differ from them not only in microstructure but in the minerals that have 



