216 V. R. VAN HISE — I * 1 ; l .-« \Mi;i;i.\N OF THE BLACK HILLS. 



Bcopic appearances, for when thin sections are examined, a glance shows 

 that their individual grains have Buffered deformation, thus accounting for 

 that exhibited by 1 1 1 « - pebbles themselves. It is probable thai slipping action 

 i- also a partial cause. 



In the purer quartzites, quartz is almosl the sole original < stituent. The 



grains are usually simple : they have not been well ass irted, varying from 

 those which are of rather small size to those in which the term " pebble" 

 might be applied. They are now usually quite angular; yet in many of 

 them, but byno'means in all, the evidence of their fragmental origin is 

 indicated by a film of inclusions about their cores. The angularity of the 

 grains is in part due to the secondary growths, but also it is in part due to 

 the mechanical action to which they have been Bubject. They generally 

 lie with tlu-ir longer axes in a common direction, and in many case- are 

 unnaturally long for ordinary erosion particles. In many of the sections is 

 included quite a quantity of black material, mostly oxide of iron. This not 

 only occurs between the fragmental grains, but is also found between the 

 Cores and the enlargements, and, what is more important, in parallel lines 

 within the cores of quartz themselves I fig. 3 . These lines are almosl univer- 



Fic 



• ii broken perpendicular to their greatest length — i. c, in the lines 



,- the im w nli ii "ii oxide. 



sally at considerable angles to the greater dimensions of the grains that is 

 divergent from the direction of schistosity : also each -rain of quartz, instead 

 of extinguishing simultaneously over its whole area, extinguishes with nynute 

 differences of orientation, the maximum variation in a single -rain ranging 

 from one to Beveral degrees, and in Borne cases reaching ten or fifteen 

 This black mat. rial, in the enlargements of the old grains and in 

 the newly crystallized interstitial quartz, is plainly a secondary infiltration 

 p roil i n i ; I ii 1 1 the material included in the original grains transverse to their 

 elongation is \\k<- this and must l>" believed to have been introduced at the 

 same time. In Bom< cases large grains have been fractured bo as to produce 



