262 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



Comparing the first three analyses (1, 2, 3), it is seen how closely they 

 agree with one another. The first has slightly less alumina and somewhat 

 more magnesia and lime. The second and third are of basalts, the second 

 being a surface flow, and the third a dike. They are rich in olivine, augite, 

 and magnetite, without hypersthene. The third contains large pheno- 

 crysts of labradorite, and carries a little orthoclase in the groundmass. 

 The first of the three analyses is of the finest-grained form of one of the 

 series of specimens from the core, and is a gabbro-porphyry. Its compo- 

 nent minerals are plagioclase with much augite, hypersthene, biotite, and 

 magnetite, and a little olivine. Although it is richer in magnesia and has 

 only two-thirds as much potash as the third rock, it has developed a great 

 amount of biotite, much hypersthene, and only a little olivine, while the 

 third rock has abundant olivine and no biotite or hypersthene. 



Comparing the next three analyses (4, 5, 6), we find a close corre- 

 spondence in chemical composition, with a smaller amount of alumina and 

 alkalies in the first and a greater amount of iron, lime, and magnesia. The 

 fifth analysis holds an intermediate place between the fourth and sixth. 

 The greatest variation is in the magnesia, which is twice as great in the 

 fourth as in the sixth. The rocks represent three very different phases of 

 consolidation. The sixth (1241) is a glass with few crystals of olivine, 

 augite, and plagioclase, and microlites of magnetite, augite, and feldspar. 

 The fourth is a fine-grained basalt-porphyry, composed of plagioclase, 

 augite, hypersthene, biotite, and magnetite, with no olivine. The fifth is 

 coarsely granular gabbro, composed of plagioclase with some orthoclase, a 

 little quartz, much augite (diallage), some hypersthene, considerable biotite 

 and magnetite, and a few crystals of olivine. 



The next two analyses (7, 8) are only a little higher in silica than the 

 previous three, and are very similar to that of the basalt-andesite glass 

 (1241) in all other respects. Thev are analyses of coarsely crystalline 

 gabbro-diorite, which grades into less siliceous rock (1388) within a short 

 distance. There are no analyses of extrusive rocks from this district 

 with the same percentage of silica with which to compare them, but at 

 Sepulchre Mountain there are andesites whose chemical composition has 

 been determined, and with which these may be compared. 



