SLATY CLEAVAGE AND ELONGATED PEBBLES. 



■Jo- 



in the eastern slates is :i broad belt of conglomerate which was discovered 

 by Carpenter. A careful study of its exposures shows that the rows of 

 pebbles and bowlders have uo regular relation whatever to the slaty cleavage 

 running across them at various angles at different localities. The pebbles 

 and bowlders themselves are, however, elongated parallel to the cleavage 

 (fig. 2). These phenomena were observed many times at points far apart. 

 Also, bedding lamination, cutting the slaty cleavage, was found at many 

 points, aud in places the former is directly transverse to the latter. It 

 follows that the breadth of the slates as measured across their outcrop gives 

 no indication of their true thickness. The fact that certain belts of 



Figure 2. — Bands of Conglomerate cutting Slaty Cleavage. 

 The elongation of the pebbles is parallel to the cleavage. 



quartzites aud schists, haviug a general resemblauce, are found parallel to 

 each other would seem to indicate that such belts are repeated by folding. 

 Within the brief time given to field study no attempt was made to work out 

 the structure of the pre-Cambrian rocks in detail ; but clearly the whole 

 question of their real thickness is thrown open. That slaty cleavage was 

 mistaken for bedding by Newton is not strange, for that schistose structure 

 and slaty cleavage not only may be, but very often are, completely independ- 

 ent of bedding was, a dozen years ago, by no means so widely recognized as at 

 present. 



Starting with Newton's ideas of the distribution and lithological distinc- 

 tions between the slates and schists, I began my study, believing that in all 

 probability the schists and granite represent an older formation than the 

 slates. Also, I supposed the two formations were either unconformable, or, if 

 in apparent conformity, were so by subsequent squeezing. 



The area about Deadwood, in the northern hills, is entirely within the 

 slate area as mapped by Newton (fig. 1). To my surprise, upon nearing 

 that place, the rocks became more aud more crystalline, and for a consider- 

 able area about this mining town the rocks are crystalline schists. Passing 



