CLASTIC ORIGIN OF THE PRE-CAMBRIAN ROCKS. 241 



The prominent features of the pre-Cambrian history seem to have been as 

 follows : The original sediments were cut by basic eruptives. They were 

 subjected to great mechanical forces applied in an east and west direction, 

 so as to produce a vertical north and south slaty cleavage before the granite 

 appeared. The resulting folding, as shown by the rows of pebble and true 

 bedding lamina?, is probably complex. After the slaty cleavage and the 

 first metamorphism in the rocks were produced, but before Cambrian time, 

 the granitic eruptions, or more properly intrusions, occurred (for there is no 

 reason to believe that any part of the granite reached the surface) in the 

 southern part of the hills. The resulting contact and dynamic action de- 

 veloped the crystalline schists and schistose structure for the most part par- 

 allel to the main igneous mass. 



Whether the crystalline schists in the northern hills were formed at this 

 same time by the intrusion of large masses of granite at no great depth from 

 the present surface, or subsequently by the later volcanics, is uncertain. It 

 is probable, however, that the latter rocks have much to do with their schist- 

 ose character, even if they were not the controlling factor. 



Clastic characters are almost indefinitely retained in fragraental rocks, 

 however old and deeply buried, unless they have been subject to dynamic 

 action. Actual movement within a rock rapidly obliterates the evidence of 

 clastic origin. 



The most important conclusion from the microscopic study is that the 

 quartzites, quartz-schists, mica-slates, mica-schists, and certain of the mica- 

 gneisses — i. e., the rocks which represent the great mass of the pre-Cambrian 

 area of the Black Hills — are of clastic origin. In the original detritus of the 

 micaceous rocks, feldspar and quartz were the predominant minerals ; but the 

 detritus of the quartzites and quartz-schists differed in that feldspar was un- 

 important. The quartzites developed from the quartzose detritus by the 

 enlargement of quartz-grains and the formation of interstitial independent 

 quartz. In proportion as they are schistose, mechanical deformation with 

 partial destruction of the fragmental particles has occurred. In the most 

 schistose phases, dynamic action has broken down the greater number of the 

 clastic grains of quartz ; yet the quartz-schists are as strong as a quartzite 

 of the ordinary type. The fracturing and cementing of the particles of the 

 rigid quartz during the movement within the rock is analogous to ice-flowage 

 in a glacier. In the mica slates and mica-schists the forces at work and the 

 results produced upon the quartz detritus have been the same as in the quartz- 

 ites. Simultaneously with this process the detriial feldspar has decomposed 

 into an interlocking mass of mica and quartz. To what extent mechanical 

 movement has helped this alteration is uncertain ; but it is known that a like 

 decomposition has occurred in rocks in which mechanical effects are slight 

 which may have suffered little interior movement, although they have been 



