RELATIONS OF FOLIATION AND BEDDING. _•>•> 



the rock, or else by their actual deformation, which latter often occasions 

 fracture. The fracture of particles occurs transverse, or at a large angle to, 

 tin 1 elongation ; for the very yielding of a grain perpendicular to the press- 

 ure, if carried far enough, ruptures it at various places transverse to the 

 direction of elongation ; i. e., approximately in the lines of pressure. All of 

 these points are beautifully illustrated by the phenomena which have been 

 described in the development of the Black Hills quartz-schists and mica- 

 schists from quartzose and feldspathic sandstones. 



The Black Hills thus furnishes one of the best instances which have come 

 to my notice of the independence of slaty cleavage and schistose structure or 

 foliation from true bedding. A great part of the Black Hills pre-Cambrian 

 rocks are of clastic origin ; yet the present prominent structures have no 

 definite relation whatever to the original sedimentation. Not only is this 

 the case, but a secondary, well-developed slaty cleavage, which locally passes 

 into genuine schistose structure, produced at the expense of original lamina- 

 tion by powerful dynamic action, has for considerable areas been itself wholly 

 obliterated by a later force, and a new 7 and more prominent foliation pro- 

 duced which cuts across the secondary slaty cleavage at various angles up to 

 perpendicularity. In rare cases a single hand specimen displays what is 

 taken to be the original sedimentation and both of the subsequent foliations 

 cutting each other nearly at right angles. Farther, associated with these 

 slaty and schistose rocks are basic eruptives which now have induced struct- 

 ures parallel to the secondary or tertiary structures of the adjacent elastics, 

 produced at the same time and by the same causes that the like structures 

 were formed in them. The principle that slaty cleavage and schistose 

 structure have very often no connection with original sedimentation is so 

 old a truth that its repetition here seems unnecessary ; yet I suspect that 

 geologists sometimes forget this important fact, wdiich should be constantly 

 borne in mind when dealing with metamorphic rocks. 



The foliation of the Black Hills slates and schists through the central part 

 of the pre-Cambrian area varies but little in strike and dip. As has been 

 before said, if this were stratification it would require a thickness of sedi- 

 ments of from 20 to 25 miles.* Under such circumstances, when no other 

 structures are found, it is common to assume that such foliation is bedding, 

 as Newton did. This requires either a belief in great thicknesses of sedi- 

 ments or else closely pressed folds, the sides of which are exactly parallel 

 and which have been truncated in such a fortunate position as to cut none 

 of the folds at their turning points. I think it may be stated as a probable 

 general truth, in cases similar to the above, that the structures are more apt to 

 be secondary than original. The strike and dip of cleavage-foliation are a 

 function of the direction of pressure ; therefore it has a uniform dip over the 



* Geology of the Black Hills of Dakota, pp. 51-52. 



