STUEGEON QUARTZITE IN FELCH MOUNTAUST RANGE. 405 



in similar crystallographic orientation with it, and that neig-hboring grains 

 thus enlarged finally interlock b}^ mutual limitation of one another's growth. 

 This explanation evidently can not account for the background of large 

 interlocking quartz areas in these rocks, for if it were true it would be nec- 

 essary to assume that the quartz grains were less numerous in the original 

 deposit than those of almost any other mineral, in some slides even than the 

 titanite or chlorite. There seems to be but one escape from the conclusion 

 that the large quartz areas must each represent a number of original frag- 

 mental quartz grains, which, as deposited, must have lain in the rock with 

 their crystallographic axes disposed entirely at haphazard; and that is the 

 hypothesis that this quartzite was not originalh' a sandstone, but consisted 

 mainly of soluble and easily replaceable material, sucli as limestone, with 

 the fragmental particles scattered through it, and that the large quartzes of 

 the background have replaced this soluble substance. I have been able 

 to find no positive evidence to support this hypothesis, and I am com- 

 pelled to believe that the rock was a sandstone in which, in some way 

 not easy to understand, considerable numbers of adjacent quartz grains 

 have united to form or have been absorbed into a new individual, leaving 

 absolutely no trace of their former separate existence. The introduction 

 of new silica, or the separation of silica from decomposing silicates in the 

 rock itself, may well have been essential factors in the recrystallization. 

 I shall make no attempt to explain the process further than to point 

 out its probable analogy with the jjrocess by which the new microclines 

 were formed in the Archean mica-schists. 



The close alignment of the clastic minerals inclosed in the large quartz 

 areas, their frequent fracture, and their occasional separation, indicate that 

 the time of crystallization probably followed a period of stress ; while the 

 very vague parallel elongation of the individuals of the background in the 

 unstrained sections would seem to show that they crystallized under static 

 conditions. Unquestionable proof of a period of stress later than the crys- 

 tallization is given by the numerous slides, in which these grains are seen 

 to have sufiFered fracture and distortion. The microscopical study of the 

 quartzites thus supplies important evidence, not aff'orded by the outcrops, 

 as to the orogenic history of the district. 



