J. P. Iddings — Origin of Quartz in Basalt. 221 



similar rocks usually do. There are no greater differences be- 

 tween the analysis of the variety without quartz and those of 

 the quartz-bearing varieties than there are between the analy- 

 ses of the three quartz -bearing varieties. So that the occur- 

 rence of the quartz in this insfance cannot be ascribed to any- 

 thing exceptional in the chemical composition of the magma. 



Chemical differences between basalts with quartz. — The fifth 

 analysis is that published by Mr. Diller in the paper already 

 cited. It shows a different chemical composition for this form 

 of quartz-bearing basalt, which is more acid than typical basalt, 

 and corresponds more closely to some andesitic forms of vol- 

 canic rocks. 



It is probable that additional analyses of other quartz-bearing 

 basalts will show as great a variation in their chemical compo- 

 sition as exists between that of basalts free from quartz grains. 



Different mineral development of chemically similar mag- 

 mas. — The sixth analysis is presented for comparison with 

 analysis Y. It is that of a magma of very nearly the same 

 chemical composition, slightly more basic, which has consoli- 

 dated under different conditions. It may serve to illustrate 

 two points : first, the mineralogical extremes to which chemi- 

 cally similar magmas may be developed. Second, the possi- 

 bility of a basaltic magma having existed at some previous 

 period in a condition of unstable consolidation, in which quartz 

 might have been crystallized out. Analysis YI is of a coarse 

 grained, quartzose diorite, perfectly fresh and unaltered, of 

 quite recent geological age, and which is composed of plagio- 

 clase feldspar, quartz, hornblende, biotite and pyroxene, with 

 accessory magnetite, apatite and zircon. The quartz is in con- 

 siderable quantity, very much more than the amount of quartz 

 observed in quartz- bearing basalts. The discussion of this 

 diorite is reserved for another paper, which is in process of 

 preparation. 



Summary. 



The principal points brought out in this paper may be briefly 

 stated as follows : 



The quartz-bearing basalt from Rio Grande Canon belongs 

 to a series of volcanic rocks, characterized by a variable amount 

 of porphyritic quartz in rounded grains. 



These quartzes are primary crystallizations from the molten 

 magma, and exhibit no definite relation to its chemical compo- 

 sition. 



Their production is to be referred to certain physical condi- 

 tions attending some earlier period of the magma's existence. 



From analogy with the occurrence of iron olivine in rhyo- 

 litic obsidian, it seems probable that the formation of primary 

 quartz in basalt took place under the influence of water-vapor 

 at a great pressure. 



