J. P, Iddings — Origin of Quartz in Basalt. 213 



by the surrounding magma. The enclosure of augite crystals 

 near the margin of grains containing glass inclusions indicates 

 that the crystallization or secretion of the quartz took place 

 in a molten magma in 'the presence of augite crystals similar 

 to those forming the enclosing shells. 



Quartz grains occur in the same manner, but to a much less 

 extent, in some of the basalts near Eureka, Nevada. They are 

 very abundant in certain basaltic rocks from the neighborhood 

 of Crescent and Whitehead Peaks and Camel Mt., Colorado. 



Possible origin of the porphyritic quartz. 



Exceptional occurrence. — The occurrence of primary quartz 

 in more or less rounded grains in basaltic rocks is exceptional, 

 and contrary to the laws which appear to govern the develop- 

 ment of the mineral constituents of volcanic rocks. The min- 

 erals developed being the result of the chemical affinities in- 

 herent in a complex solution of silica, alumina, with certain 

 alkaline earths and alkalies (mainly iron, lime, magnesia, soda 

 and potash), between the various demands of which there 

 must be a mutual accommodation, it is evident that the re- 

 sulting minerals must be those which under the conditions 

 attending their crystallization satisfy all the chemical demands 

 imposed by those conditions. 



Limited variation of conditions. — That a change in the con- 

 ditions under which crystallization takes place in a mixed solu- 

 tion affects the nature of the crystallization is well known, 

 and has been demonstrated experimentally. From the gener- 

 ally uniform nature of the crystallization of most volcanic 

 rocks, the correspondingly uniform conditions attending their 

 crystallization are indicated. Where variations in the pro- 

 ducts of crystallization occur, the conditions effecting them 

 may often be recognized. Hence, variations from the general 

 order and nature of crystallization within certain limits have 

 become generally accepted facts, so that no one expects the 

 mineral composition of rocks to be in rigid accord with their 

 chemical composition within these limits. 



An exception emphasizing greater variations. — While in 

 the great majority of cases the mineral composition of vol- 

 canic rocks indicates a generally uniform range of conditions 

 which must have attended their consolidation, yet there are 

 exceptions to the ordinary grouping of minerals in these 

 rocks that emphasize the influence of certain attendant condi- 

 tions, which must be regarded as physical. An instance of 

 this is the mineral association in the lithophysse of the rhyo- 

 litic obsidian from Obsidian Cliff, Yellowstone National Park, 

 and from Cerro de las Navajas, Mexico. These holocrystalline 



