100 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



fig 2. In the seventh grade from the coarsest end the feldspars range from 

 1.25 mm. to 0.5 mm., and the quartzes are about 0.12 mm. 



As the grain becomes smaller, idiomorphic forms are more numerous, 

 especially of hornblende and biotite. And these are more idiomorphic 

 when quartz is more abundant, since they are always idiomorphic Avith 

 respect to this mineral (PI. XIX, fig. 3). A gradual change of this character 

 can be followed to grade 26 of the table, where the average size of grain 

 is 0.23 mm. Here there is a slight tendency to porphyritic structure, which 

 does not show megascopically. Porphyritic structure becomes noticeable 

 in grade 24 (PI. XIX, fig. 4), and still more so in grade 14. This has a 

 groundmass composed of grains of feldspar and quartz with poorly defined 

 outlines, besides microscopic pyroxenes and magnetites. The phenocrysts 

 are lime-soda feldspar, hypersthene, augite, and some irregular patches of 

 biotite. There is no hornblende. The finest-grained forms of these rocks, 

 grades 17 to 13, have similar structures, and might be called pyroxene- 

 andesite- porphyry. 



This group of rocks presents a continuous series from fine-grained 

 andesite-porphyry, with phenocrysts of hypersthene and plagioclase, to 

 coarse-grained hornblende-rnica-diorite with a variable percentage of 

 jwroxene. In two instances the transition is represented by specimens 

 collected within short distances of one another. Thus, Nos. 268 to 271, 

 279, and 287 (PI. XIX, fig. 3) were 1 foot apart in a continuous rock 

 mass, and No. 266 is from the same mass. Nos. 277, 278, 281, 284, and 289 

 are from one rock mass exhibiting a gradual change of grain through a 

 distance of 4 feet. No. 267 is from the same mass. 



The microscopical characters of the constituent minerals are much the 

 same throughout the series, but there are certain features that vary with 

 the coarseness of grain of the rock. 



The feldspars are mostly labradorite, and to a less extent andesine. 

 The idiomorphic crystals and the zonal portion of the allotriomorphic ones 

 increase in size as the grain of the rock becomes larger. Their twin lamellae 

 become broader, the number of inclusions of pyroxene and other ferromag- 

 nesian minerals and of magnetite diminish with increasing grain, and the 

 abundance of minute dots and needle-like inclusions increases. The 

 feldspars, forming irregular grains in the groundmass of the andesite- 

 porphyries, crystallize as a border around the idiomorphic feldspars in the 



