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



The red, vesicular variety also bears many porphyritic grains 

 of quartz. On the surface of one side of a small specimen 

 7'5 cm long by 6 cm wide, there are 75 grains, and they are not 

 especially abundant in this particular specimen. 



In thin section these red varieties resemble the general 

 microstructure of the gray, compact ones, but they do not 

 appear to be noncrystalline ; there is probably a little glass 

 present. The rocks are filled with red oxide of iron, which 

 also colors the margins of the porphyritic olivines. They con- 

 tain a little more augite than do the gray varieties. Most of 

 the quartz grains have dropped out in grinding, but fragments 

 remain, and the augite rings indicate where they were once 

 located. 



As to the nature of the quartz which forms these porphy- 

 ritic grains, it is evidently not an alteration product of other 

 minerals nor an infiltration product, for the rocks are quite 

 fresh and exhibit a very slight alteration on the surface of the 

 olivines. On "the contrary, the quartz grains undoubtedly 

 existed in the rock-mass in their present form previous to the 

 final consolidation of the magma. For each grain is closely 

 surrounded by a shell of augite crystals, intimately connected 

 with the enclosing rock-mass. 



This augite shell forms a narrow border or ring in thin sec- 

 tion, and is composed wholly of crystals of augite radiating 

 from points along the side of the rock-mass toward the quartz. 

 The augite crystals crowd against the surface of the quartz 

 grains, but there is no line of demarcation between them and 

 the rock-mass : the outside augites lie among the feldspar and 

 magnetite individuals and take part in the general structure of 

 the rock. In the coarser grained varieties the feldspars some- 

 times enclose a number of the augites situated on the outside 

 of the shell, thus demonstrating that the augite shell existed 

 prior to the final consolidation of the rock. 



The substance of the quartz composing these grains is per- 

 fectly pure, and free from inclusions of gas, fluid or glass ; in 

 one instance there was a minute crystal of zircon. Each grain 

 is a single individual, with uniform optical orientation through- 

 out its substance. Occasionally two individuals are in juxta- 

 position. But they are never made up of aggregates of small 

 grains, the form which secondary quartz usually assumes. 

 The grains are rounded or subangular. 



The substance and shape of the quartzes are* like those of 

 the porphyritic quartz grains in other volcanic rocks when 

 they are free from inclusions, as often happens in rhyolites. 

 They are not like the quartzes of granites and gneisses or of 

 sandstones, which are more or less tilled with inclusions of gas 

 and fluid, and frequently with individualized inclusions. 



