BASALT FLOWS. 239 



Pirsson, who determined it to be mordenite. 1 Very glassy forms of rock 

 are found in the breecia on the ridge south of Indian Peak. Some are 

 fragments of black glass with a few small phenocrysts of tabular feldspar. 

 Others are gray and exhibit spheroidal cracking, and constitute masses of 

 considerable size. The black fragments are basalt-andesite glass, which is 

 dark brown and almost opaque in thin section, with few scattered microlites. 

 In some sections the glass is mottled and streaked with light brown. The 

 microlites consist of feldspar needles and grains of magnetite surrounded 

 by halos of colorless glass, besides a few small augite crystals and serpen- 

 tinized olivines. There are somewhat larger plagioclases with inclusions of 

 brown glass. The chemical analysis of this rock (analysis 6 on page 260) 

 proves it to be intermediate between basalt and andesite. The gray glassy 

 varieties belong to pyroxene-andesite. In thin section this glass is colorless 

 to light brown, with small crystals of plagioclase and fewer of magnetite, 

 augite, and hypersthene, and in rare instances hornblende. 



The variety of basalt with feldspar phenocrysts 30 mm. long is char- 

 acterized by a slightly different microstructure. The groundmass consists 

 of tabular feldspars, composed of kernels of labradorite with marg'in of 

 orthoclase, besides smaller augites and magnetite, through which are scat- 

 tered larger microscopic crystals of the same minerals, with patches of 

 serpentine. The phenocrysts are small megascopic labradorite, augite, 

 decomposed serpentine and magnetite, besides extremely large tabular 

 labradorite, with abundant inclusions of glass or groundmass. These basalts 

 are intermediate between normal basalts and the shoshonite described in 

 Chapter IX. 



BASALT FLOWS. 



The lava flows intercalated in the breccias are all basalt, with variable 

 amounts of olivine, judging from the thirty specimens of them which were 

 collected. None proved to be andesite. In general they come from higher 

 parts of the volcano, and represent later phases of its eruption. But some 

 of them occur among the earlier products, and while it may be said that 

 the basalts formed almost the whole of the later outflows of the volcano, 

 and that the magma became more basic up to this period, it must not be 

 forgotten that the eruptions varied constantly within narrow limits, and that 



'On mordenite: Am. Jonr. Sci., 3d series, Vol. XL, Sept. 1890, pp. 232-237. 



