CONGLOMERATES METAMORPHOSED TO GNEISS. "215 



Resting unconforniably upon the Black Hills slates and schists is the 

 Potsdam sandstone, which is locally a quartzite. The induration of this 

 rock has been found by Crosby to be due to the deposition of interstitial 

 silica. He does not find, however, in general that it has coordinated itself 

 with the original grains. My own sections, upon the contrary, show this 

 to be the case in the quartzites collected by us. 



The Conglomerates and Quartzites. — No microscopic study has heretofore 

 been made of the character of the changes which the various minerals have 

 undergone in the quartzites, conglomerates, slates, and schists of the pre- 

 Cambrian area, although Caswell gives their mineralogical composition.* 

 In tracing out the series of changes I begiu with those rocks which are 

 nearest to their original condition, the quartzites and conglomerates aloug 

 Box Elder creek, in the northeastern portion of the pre-Cambrian area. 



This conglomerate area has been mentioned by both Carpenter and 

 Crosby. It extends several miles along the creek, and has a very con- 

 siderable breadth. The conglomeratic bands alternate with those which are 

 non-conglomeratic. The bowlders, oftentimes more than a foot in length, 

 are at times very abundant. They vary from this magnitude to those which 

 are so small as to be lost in the matrix. This conglomerate has been sub- 

 jected to powerful dynamic action. This is evident from the fact that the 

 pebbles and bowlders are elongated in a common direction, in some cases 

 the longer diameters being three times as great as the other dimensions. 

 These elongated pebbles often, instead of having roundish terminations, end 

 in sharp points. Also, in many cases, the pressure has been so intense as to 

 merge the pebbles into each other. In certain places the process has gone 

 so far as to almost wholly destroy the pebbles, it being only possible to dis- 

 cover them upon a polished surface transverse to the plane of schistosity. 

 Cleaved parallel to the foliation or broken, these conglomerates appear to 

 be but a coarse schist. The pebbles and matrix are practically one. This 

 extreme alteration is most frequent with the finely conglomeratic phases. 

 These betray no evidence of their fragmental origin, and taken by themselves 

 would be regarded as ordinary crystalline schists. Some of them have all 

 the characteristics of a coarse foliated gneiss. The associated conglomerates 

 only indicate that these rocks were originally clastic. 



The more purely quartzitic bands do not macroscopic-ally so plainly show 

 the action of the forces to which they have been subject. Crosby and 

 Carpenter both noted the elongation of the pebbles of this conglomerate 

 but they agree in the statement that the grains themselves have not 

 suffered by the deforming action. They explain the present elongated 

 nature of the pebbles by supposing the grains to have slipped over each 

 other. These statements must have been wholly based upon the maero- 



* Geology of the Black Hills of Dakota, pp. 471-ls:i. 



