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



Another group of volcanic rocks, specially characterized by 

 abundant rounded grains of quartz, occurs in the vicinity of 

 Crescent Peak. Colorado. The group embraces basalt, ancle- 

 site and possibly trachyte. These all bear rounded grains of 

 quartz, and some of the olivine- bearing varieties also carry 

 hornblende paramorphs, which furnish additional evidence of 

 a change of physical condition from one which induced the 

 crystallization of certain minerals, to a later one, in which they 

 were partially resorbed. 



Chemical similarity of 'basalts with and without quartz. — 

 That the chemical composition of quartz-bearing basalts is not 

 characteristic of a particular modification of rock magma will 

 be seen from the accompanying analyses : 



I. II. III. IV. T. VI. 



SiOo 



52-27 



52-37 



5 1 '57 



52-38 



57-25 



56-28 



TiOo 



1-49 



1-60 



1-43 



1-22 



0-60 



0-84 



A1 2 3 .__ 



17-68 



17-01 



17-72 



1879 



16-45 



14-23 



Fe 2 3 ___ 



2-51 



1-44 



6-24 



2-88 



1-67 



4-69 



FeO .... 



5-00 



5'89 



1-78 



4-90 



4-72 



405 



MnO_._. 



0-23 



0-32 



0-45 



0-18 



o-io 



0-16 



CaO .__. 



839 



7-59 



8-82 



7-70 



7-65 



7-94 



MgO___. 



6-05 



686 



4-91 



4-9 L 



674 



637 



BaO 



006 



0'06 



0-16 



011 



o-oo 



SrO tr. 







£ 2 .... 



1-58 



1-59 



1-99 



1 76 



1-57 



1-23 



Na 2 ... 



4-19 



3-51 



3-59 



399 



3-00 



2-98 



Li 2 .... 















o-oo 



o-oi 



HoO .... 



0-S2 



1-29 



0-64 



0-53 



0-40 



0-93 



CO, 



tr. 



0-37 



0-58 













p 2 6 5 











0-56 



0-20 



0-40 



Cl 



tr. 



tr. 













0-17 



so 3 



---- 





— 



— 



... 



tr. 



100-27 99-90 99-88 9991 100-35 100-28 



I. Quartz-bearing basalt, Rio Grande Canon. X. M. (L. O. Eakins.) 



II. 

 III. 

 IV. Basalt without quartz, " " ;t " 



V. Quartz-bearing basalt. Cinder Cone. Lassen's Peak, Cal. (W. F. Hille- 



brand.j 



VI. Quartzose diorite. Electric Pk., Yellowstone Park. (J. E. Whitfield.) 



The first three are of three forms of quartz-bearing basalt 

 from Rio Grande Canon : the first is a light gray dense basalt ; 

 the second, a greenish black, dense basalt ; and the third, a 

 dark red vesicular basalt. They have practically the same 

 composition with slight variations. The higher oxidation of 

 the iron in the red rock is indicated by the high percentage of 

 ferric oxide in analysis III. 



The fourth analysis is of a gray, dense basalt from Rio 

 Grande Canon, which resembles the basalt from which the first 

 analysis was made, except that it exhibits no quartz, either in 

 macroscopic or microscopic grains. These four are normal 

 basalt analyses resembling one another as closely as analyses of 



