J. P. Iddings — Origin of Quartz in Basalt 211 



It is evident, therefore, that the quartz grains in these basalts 

 are primary constituents of the rock ; that they are more like 

 the porphyritic quartz secretions of other volcanic rocks in 

 their microscopical habit than like fragments of quartz from 

 granitic or gneissic rocks or sandstones. 



Moreover their mode of occurrence in these particular 

 basalts militates against the theory that they may be fragments 

 of foreign rocks which have been caught up by the basalts dur- 

 ing their eruption. For they are uniformly and intimately 

 distributed through each of the seven hand-specimens which 

 were collected from different parts of a large basalt flow, and 

 represent portions of it which differ in crystallization and gen- 

 eral habit. Furthermore, the grains are isolated individuals 

 and not clusters and never exhibit indications of having been 

 crystallized together with other, minerals. They have every 

 appearance of being primary secretions or crystallizations from 

 the rock magma. 



Similar occurrences in other localities. 



Mr. J. S. Diller (this Journal, Jan., 1887) has described the 

 occurrence and distribution of very abundant quartz grains 

 throughout the recent basalt flow near the Cinder Cone, ten 

 miles northeast of Lassen's Peak, California. And his argu- 

 ment for the primary and essential nature of the quartz in this 

 particular basalt appears to me to be conclusive. 



Two new occurrences of quartz-bearing basalt may be de- 

 scribed in this connection, as they go to show that such ba- 

 salts are scattered over a large area of country, having been 

 found in California, Nevada, Arizona, New Mexico and Col- 

 orado. 



That from Arizona is a fine example of quartz-bearing ba- 

 salt ; it is a red compact rock from the canon near granite 

 tanks in the vicinity of Santa Maria Basin, and was collected 

 by Mr. Clarence King. In the hand specimen the only por- 

 phyritic secretions are rounded grains of glassy quartz from 

 one to eight millimeters in diameter, together with somewhat 

 smaller olivines. The quartz grains are so numerous that one 

 small specimen about 6 cm. square shows as many as 30 

 grains on all its surfaces. 



In thin sections the basalt is glassy, with abundant lath- 

 shaped plagioclase and much red oxide of iron scattered 

 through it, which obscures the other constituents. The ground- 

 mass bears small porphyritic crystals of light green augite and 

 olivine. In the dense specimens, the olivine is partly fresh 

 and colorless, partly colored orange and red. In the porous 

 specimens, the olivine has been entirely removed, leaving its 



