BASALT. 391 



found to be transparent with very dark borders. This ghiss base and the micro- 

 structure of th(^ rock as a whole are the only variable factors in this otherwise nionot- 

 onous group of rocks. The variations in these features will be separately mentioned 

 in the following notes. Thin sections 253, 200, 2G1, 250, 257, 258, and 202 are fnmi the 

 flow of ba.salt east of the town of Eureka, at the western base of Richmond Mountain. 

 Xo. 253 is vesicular, with relatively great irregularity in the size of the crystals, a small 

 amount of glass base, and, what is generally noticeable, a marked tlow structure. Xos. 

 200 and 201, which are closely related in the field, have a more even grain and compar- 

 atively little glass, with few globulitcs and acicular niicrolites. The first is vesicular, 

 with some cavities filled with the delessite like mineral and possibly the trace of 

 olivine. The second is compact; the mottled appearance of its groundmass is not 

 traceable to any modificatiou of the crystalline structure and must lie in the isotropic 

 base. Nos. 250 and 258, from compact and vesicular portions of the same flow some 

 distance fi'om the last two, are alike in structure, being of uneven grain with numer- 

 ous larger aui^itc crystals similar to No. 253. The glass is (lartly brown, which is 

 also the case m .':in section 257, which is exceptionally beautiful, having a coarser 

 and more even texture with a larger anioiuit of glass (Fig. 2, PI. vii). Thin section 202 

 differs from all the rest in having little or uo glass and, besides the lath-shaped feld- 

 spar microlites, possessing ill defined feldspar grains approaching a microcrystalline 

 structure. The basalt on the summit of Richmond Mountain, thin sections 204, 205, 

 207, 208, has the same modifications of its structure. Nos. 204 and 205 are rich in 

 colorless glass filled with the minutest globulites, which give it a brown color. The 

 crystalline constituents are very small ami almost wholly augite and magnetite. The 

 crystals of 207 are larger and include more feldspar, which is also true of 2(iS, where 

 the mottled appearance of the rock is due to the uneijual distribution of the augite 

 gr;iiiis. In these four thin sections augite is greatly in excess of the feldspar. A 

 fine thin section with rich brown globulitic glass and minute augite crystals, closely 

 resembling No. 204, is from a bowlder on the foothills to the north of the mountain 

 (270). No. 209 is a red, finely porous variety fiom near the summit of Richmond 

 Mountain. Its I'ed color is the result of the partial decomposition of abundant olivine. 

 The rock is more highly crystalline than the neighboring basalt. It is rich in feldspar 

 and poor in slightly globulitic glass. Of the basalt exposures in the pumice at the 

 south base of Richmond Mountain, the luost westerly dike exhibits fine columnar 

 structure. Its thin section, 271, shows it to be quite like No. 200, in being well 

 crystallized, with little glass and considerable feldspar, and a very little olivine. 

 That from immediate coTitact with pumice on the spur south of the sumnut, 273, is 

 finely vesicular-, poor in crystals, and full of ferrite, resembling those portions of 

 basalt found included in several altered pumices. Thin section 274 is from one of 

 the coarser grained, more highly crystallized varieties, which is poor in glass, but 

 very rich in augite, and having a trace of olivine in the form of a small number of 

 characteristically decomposed sections. 



