J. P. Iddings — Origin of Quartz in Basalt. 209 



exhibit a remarkable Dumber of porphyritic grains of quartz 

 and to offer some explanation of its occurrence. 



The basalts in question occur in the vicinity of Rio Grande 

 Canon. • Those which form the second and fourth ledges from 

 the top of the wall of the canon are very much alike ; they are 

 light gray, fine grained, dense rocks with small porphjritic 

 crystals. In thin section thej are found to be holocrjstalline, 

 with much lath- shaped plagioclase and less augite in grains and 

 stout crystals, besides magnetite and much colorless olivine in 

 crystals and grains. There are a few porphjritic crystals of 

 olivine and augite, and very abundant microscopic needles of 

 apatite. 



Seven other specimens of basalt were collected from a large 

 flow, covering the country for four miles; two of them are 

 light gray and dense ; three are greenish black and dense ; and 

 two are dark red and vesicular. All are very fine grained. 



The light graj basalts are somewhat like those forming the 

 two ledges on the canon wall, but carrj more porphjritic oli- 

 vines, besides abundant, small grains of crackled quartz. 



These quartz grains are surrounded bj light green shells, 

 which are composed of microscopic augites. In some cases the 

 shells cover the quartz grains 'on the surface of the rock ; in 

 others, especially on weathered surfaces, the quartz has dropped 

 out, leaving the augite shell adhering to the rock. The quartz 

 grains are distributed through the rock quite as uniformly, 

 though not so abundantly as are the crystals of olivine. 



In thin section these two specimens of basalt are alike and 

 resemble those from the canon wall, except that thej are 

 slightlj coarser grained. Thej are holocrjstalline, and are 

 composed of lath -shaped and tabular plagioclase, light violet- 

 brown augite in crystals and grains, with magnetite and much 

 olivine in grains and porphyritic crystals. None of the quartz 

 grains appear in the thin sections of these two specimens, but 

 the augite rings and clusters indicate where they were located 

 before the grinding of the sections. 



The three specimens of greenish black basalt also exhibit 

 macroscopic olivine and abundant quartz grains. The quartz is 

 more compact and freer from cracks than in the previous speci- 

 mens. 



In thin sections, this form of the basalt has the same micro- 

 structure as those just described, but is finer grained and con- 

 tains more augite. The olivine is partly altered to dark green 

 serpentine. In one of these thin sections there are seven 

 grains of quartz or indications of their former presence, in 

 another section two, and in the third there are ten. 



Am. Jour. Sci.— Third Series, Vol. XXXVI, No. 213.— Sept., 1888. 

 J 4 



