PETROLOGY IN AMERICA. 469 



Weed and Pirsson, and seems to be the porphyritic or 

 "dyke" equivalent of that plutonic rock. A rock with 

 porphyritically developed olivine and augite has a peculiar 

 composition. With 47 per cent, of silica it has as much as 

 21 per cent, of magnesia, and on the other hand 3 per 

 cent, of alkalies, chiefly potash. Iddings has remarked that 

 this rock falls under the type which he has named absarokite. 

 The same remark applies to others from South Boulder, 

 Antelope, and Cottonwood Creeks, which Merrill has 

 described under the title of lamprophyres. These contain 

 mica as well as olivine and augite, but, as before, no por- 

 phyritic felspar, while the analyses show that the felspar 

 of the ground-mass is largely of a potash-bearing species. 



In his Origin of Igneous Rocks (1892), Iddings drew 

 attention to certain dykes and lava-flows of exceptional 

 character occurring in the Absaroka Range in the Yellow- 

 stone Park region. These rocks belong to a late stage in 

 the igneous activity of the district. While showing evident 

 consanguinity with the more usual types with which they 

 are associated, they have chemical and mineralogical 

 peculiarities comparable with those of Merrill's rocks in the 

 country farther north. In a later paper Iddings (14) has 

 given a more complete account of these rocks, which con- 

 stitute what Brogger styles a "rock-series," that is, a num- 

 ber of types representing like phases of differentiation from 

 what may be regarded as the more normal series of basalts 

 and andesites, with which they are associated. Iddings 

 distinguishes three types, absarokite, shoshonite and bana- 

 kite. They are usually porphyritic, the phenocrysts being 

 of olivine and augite, with labradorite in the two latter 

 types. The ground-mass is rich in alkali-felspars, and in 

 some varieties contains leucite. In absarokite, which is the 

 most basic type with 46 to 52 per cent, of silica, there is no 

 porphyritic labradorite but only olivine and augite. In 

 shoshonite, with a silica-percentage of 50 to 60, and com- 

 paratively rich in alkalies, labradorite figures among the 

 porphyritic elements, and in banakite it predominates. In 

 this type mica partly takes the place of augite in the ground- 

 mass, and the rocks are highly felspathic. Silica ranges 



