218 c. i;. van 1 1 1 - 1 : — pre-cambrian of tin: black hills. 



Bchistose phases of the quartzites differ from the leasl schistose phases in the 

 degree to which this process has been carried out. 



All the foregoing facts are explained if it be assumed thai these rocks 

 have been subjecl i" so greal :i pressure thai movemenl has occurred within 

 them, elongating :ill the ^mins, destroying the perfection of the orienta- 

 tion in the particles or actually breaking them down altogether. Their 

 elongation transverse t<> fracture and in the lines of pressure, with the intro- 

 duction of inclusions along the cracks, are just the phenomena which would 

 l.e expected from tin- well-known mechanical experiments on minerals and 

 rucks by Daubree and ( >. Lehmann. I f there was no macroscopic evidence 

 that these quartzites and conglomerates had been subjecl to powerful 

 mechanical action, the microscopic evidence would be conclusive upon this 

 point. 



The presence of parallel line- of inclusions, both solid and fluid, running 

 continuously acn tions is a matter of some interest (figs. 3 and I I. 



That the rock.- showing this are clastic and the inclusions secondary in the 

 Black Hills is indisputable. .Mechanical action has cracked the grains in 

 parallel planes. These cracks have become filled with liquid. Later, by 

 the deposition of quartz, they have again become cemented and retain at times 

 numerous liquid inclusions. Cohen,* in his memoir upon the rocks of the 

 Oben Weilerthal, argues thai a certain quartz-schist is noi of clastic origin. 

 He brings as proof againsl this the presence of a greal number of pores 



which are liquid filled, arranged in straight lines running from ■ -rain to 



another. It is evidenl thai this appearauce has not the force which he as- 

 signs to it. At various times the presence of liquid filled cavities has been 

 taken as indicating the origin ol the quartz in which they were contained, 

 •■line maintaining thai such quartz cannol be igneous, bul must be of 



metamorphic origin or formed by aqueo-ig us fusion. It is evident. Bince 



such inclusions may be Be< lary, thai this phenomenon cannot be used to 



explain the origin of a quartz. 



The pebbles and bowlders of the conglomerates are usually either simple 



or complex fragments of white quartz, which could have 1 q derived from 



vein- or from a coarse granitic rock. Some of the quartz pebbles, however. 

 both in hand specimens and under the microscope, appear themselves to be 

 offragmental origin. This clastic appearance, if noi deceptive, would indi- 

 cate that there weir breaks in the deposition of the series and that an earlier 

 formation yielded detritus to a later, or else thai before these ancienl crystal- 

 line -late- and quartzites were deposited there existed other fragmental Beries 

 from which the\ obtained a portion of their detritus. Crosby and Carpenter 

 speak of the materials of the conglomerates as having been derived from t he 

 crystalline Bchists and granites to the southwesl ; upon what evidence, how- 



llungi 



ii in iiiK'-n. Band 1 1 1 188 



