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



characteristic outlines marked- by iron oxide. The augite is 

 more or less reddened. 



The quartz is the same in both varieties. It forms rounded 

 and subangular grains, of very pure substance, almost free 

 from inclusions. The few inclusions observed consist of color- 

 less glass, in one instance gas, besides a small zircon and an 

 apatite. 



In only a few instances do the quartz grains possess a con- 

 tinuous shell of augite crystals. Most of the grains are 

 bounded directly by the groundmass of the rock, or have a 

 fragmentary augite shell which is sometimes separated from the 

 quartz grain by red glass, sometimes by a strip of groundmass 

 whose flow structure indicates that it has forced its way be- 

 tween the augite shell and the quartz grain. 



Fragments of augite shells are observed at some distance 

 from the quartz, or even entirely isolated in the rock 

 mass, from which it appears that the quartz grains were at 

 one time surrounded by a shell of augite, as in most other oc- 

 currences of quartz-bearing basalt, but that in this instance 

 the subsequent movement of the magma broke the shells and 

 dislocated them. 



The second occurrence to be described is that of a dark 

 colored, fine-grained basalt from Elk Head Creek, at the south- 

 east base of Anita Peak, 15 miles northeast of Hayden, Col- 

 orado, in which porphyritic quartz grains are very abundant. 

 Dr. Whitman Cross kindly furnished me with the thin sec- 

 tions of this rock for study and description. They present 

 two modifications of the basalt, and show that the rock is 

 partly altered, the olivines having been converted into ser- 

 pentine, which is disseminated through the rock. The rock 

 resembles the basalt from the Eio Grande canon, 1ST. M., in 

 mineral composition and structure, but the quartz grains, 

 which are very abundant, have a somewhat different micro- 

 scopical character. They are surrounded by an augite shell 

 in every case. But the shell is quite thin and its connection 

 with the quartz substance is more intimate. In places the 

 augites penetrate the quartz substance. In some instances the 

 quartz exhibits sharp-edged crystal boundaries. The quartz 

 grain is sometimes made np of two or three individuals crys- 

 tallized together. The inclusions are numerous and consist 

 of gas cavities and less abundant glass inclusions, besides zir- 

 con and apatite. The microscopical character of the quartz re- 

 sembles that of certain porphyries. 



Its primary nature is shown by the presence of glass inclu- 

 sions and the encircling shells of augite, and by its uniform 

 distribution through the rock. The existence of crystal bound- 

 aries shows that there was little if any resorption of the quartz 



