PETKOCRAPHICAL CHAIIAGTEE OF MESNAIM) QFAETZITE. 227 



quartz, kadliu, and stn-icite, tlie tir.st being- often predominant. Numerous 

 veins of secondarA' (juartz cut the matrix and tlie coarser g-rains. The 

 gneisses adjacent differ from the clastic rocks just described in that distinct 

 residual, although much altered, feldspar remains, in the absence of abunchxnt 

 secondar\- chertv ([uartz, and in the distinct granitic texture. 



(3) The interstratified conglomerates differ from (2) only in that the 

 predominant pebbles are chert, jasper, quartz, and ferruginous schists, and 

 that granite pebbles are sparse or absent altogether, although sometimes 

 much detrital feldspar is present. 



The pure quartsitcs grade through a feldspatliic (piai-tzite into the fine- 

 grained conglomerates. The least mashed phase of the (^uartzites consists 

 almost entirelj of well-rounded, uniform grains of quartz of medium size, 

 which have become enlarged, the enlargements interlocking and nearly 

 tilling- the interspaces. A very small amount of sericite, oxide of iron, and 

 independent secondary quartz is seen between the grains. In certain less 

 pure phases larger amounts of these materials are ])resent. Many of the 

 least masheil ([uartzites show remarkalde pressure effects The grains Avhich 

 have been least affected show merely undulatory extinction. From this 

 phase the grains grade into those in which minute cracks have foi-med 

 However, whetlier the extincti(tn is undulatory <ir there are distinct cracks,, 

 the breaking has been in two directions at right angles to each other. The- 

 fractures in one direction may be more marked than those in the other, and 

 one set may disappear. Where the fracturing is distinct, eacli of the quartz 

 grains is broken into a large number of parallel plates, or, if fractured in 

 two directions, into a \ery large numl)er of minute rectangular blocks. 

 These fractures are jilaiuly produced in the shearing planes.^ That thev in 

 many cases can not be quartz cleavage is shown b)^ the fact that they pass 

 in the same direction from grain to grain. Where the fracturing is most 

 marked iron oxide and gas and water bubbles have formed in the openings. 



The pure vitreous quartzites also pass into the cdiert}- quartzites. In 

 these the dynamic effects upon the original quartz grains are more pro- 

 nounced. Between the original grains and through them there has been a 



' See Principles of North Americaa pre-Cambrian geology, by C. R. Van Hise: Sixteentb Ann. 

 Kept. U. S. Geol. Survey, Part I, 1896, pp. 69G-698. 



