232 R. VAN WISE — l»RK-CAMBRIAN "1 Till. BLACK HILLS. 



:i :i direction :it righ! angles to it. As the granite is approached, 

 the slaty cleavage becomes fainter, the schistosity becoming gradually more 

 rtinct, and near the granite the cleavage is wholly replaced by the schistose 

 structure. The change of the original slaty cleavage to :i Bchistose structure 

 -: and west of the granitic area did nol result in a variation of the direc- 

 tion of foliation, as the new force was parallel t'> the old; l>ut Bouth of the 

 - on the north, the new foliation is al right angles to the older Blaty 

 clea 



iponding to the double cleavage of the rocks uorth of the granite is a 

 n liar arrangement of the mica-folia as seen in thin section. In general, in 

 the slates and schists, the micas are arranged with their basal cleavage parallel 

 to ilit- slaty parting or Bchistose structure, li is to be expected when the first 

 of these structures yet remains and the second also has developed in a new di- 

 tion, that a double arrangement of the mica would be found ; and Buch is the 

 Dhis phenomenon is best shown in those rocks in which the two struc- 

 tures are about equally prominent aud at right angles to each other. Here the 

 larger mica Bakes are parallel to the slaty cleavage, while the smaller and more 

 numerous ones arc parallel to the schistose st ructure I fig. 1 . plate 5 i. This 

 curious arrangement corresponds with the genesis of the minerals as worked 

 out. The Blaty cleavage is earlier than the Bchistose structure, and folia of 

 mica had developed with bases parallel to the former before the latter ap- 

 peared. At the granitic eruption the new mica flakes arranged themselves 



in corresp lence w ith the developing schistosity. It appear.- as if this later 



force al a distance from the granite was nol sufficient to rotate the mica par- 

 ticles which had already formed. They continue. 1 to grow, and reached a 

 eater magnitude than the newer folia parallel to the schistose structure. 

 In the most crystalline schists adjacent to the granite the new force was 

 able to wholly obliterate all the effects of the previous slaty cleavaj 



Bj NG, < 'll W \'.| \\ h l'< HI \ I l"\. 



I he foregoing Btudies of the quartz-schists, mica schists and mica-guei 



• srvations on the production of slaty cleavage and foliation. 

 \-i- well known, these structures develop as a consequence of dynamic 

 action. This results in the arrangemenl of the original and secondary 

 particles with their two greater dimensions perpendicular to the lines of pri- 

 mary force, producing cleavage or foliation in the planeof these dimensions. 

 i- a linear-parallel arrangement of the particles in this plane 

 ponding to the direction of a secondary force. The work is accom- 

 plished by the development of new minerals, which arrange themselves perforce 

 with i hen longer axes in the directions of least resistance ; aud, so far as the 

 original part mcerned, either by their rotation in the flowage of 



