SOME DYNAMIC PHENOMENA. 349 



the fold. That is, dynamic metamorphism ought not to be so 

 extensive in the south range as in the north range. The facts 

 described by Irving/ and those noted by me, fully agree with 

 this anticipation. The central parts of the heavy, little inclined 

 beds of the south range are largely indurated by simple enlarge- 

 ment. The pressure has not been sufficient to obliterate the 

 cores, but has apparently granulated the exterior of some of the 

 larger fragments, as in hand specimens the exteriors of the large 

 blue quartz grains are white. Very generally the grains show 

 slight wavy extinction. A few of them are distinctly cracked. 

 The crevices thus formed and those in the interstices have been 

 filled in large part by infiltrated silica, but their positions are 

 plainly indicated by difference in extinction, by bubbles, by iron 

 oxide, or by secondary mica which has taken advantage of the 

 minute crevices. 



However, as described by Irving, between the heavy beds of 

 quartzites are often layers, cut by a diagonal cleavage which 

 dies out in passing into the thick beds. The layers showing 

 cleavage sometimes pass into those showing the beginning of 

 foliation, the rock then nearing a schist. In the centers of the 

 schist zones, the schistosity approaches parallelism with the 

 bedding, and in passing outward curves from this direction until 

 it crosses the bedding at an angle, at the same time becoming 

 less marked and grading into ordinary cleavage, which dies out 

 in the quartzite. Upon the opposite side the transition is of the 

 same character, but the curve is in the opposite direction. 



Irving apparently regarded these shear zones as originally 

 beds of a different character from the adjacent quartzite, and his 

 conclusion is fully borne out by the thin sections. The micro- 

 scope shows that the^ grains of quartz are of small size, and 

 separated to a greater or a less extent by interstitial clayey 

 material. Because of this partial separation of the grains of 

 quartz, they have not been granulated to the extent that one would 

 expect from the schistosity of the rock, most of the original 



^The Baraboo Quartzite Ranges, by R. D. Irving. In Vol. II, Geol. of Wis., pp. 

 510, 516. 



