Plate IX.— BRECCIATED WEWE SLATE. 



Fig. 1. Shattered Wewe slate from the NE. J sec. 21, T. 17 N., R. 26 W. (Atlas Sheet XXXII). The 

 oherty slate was shattered hy a, first movenieut which opened cracks In various directions. 

 These were filled with secondary rjiiartz. The rock was again shattered, and the openings 

 thus formed were filled by secondary quartz, limonite, and hematite. Besides this shat- 

 tering there was movement betweeu the individual mineral particles, which granulated the 

 rock. In the interspaces between the particles chert and hematite were deposited. By 

 observing the figure closely innumerai)le miuiitu brilliant flecks of the latter may be seen. 

 Natural size. 



Fig. 2. Brecciated Wewe slate from the same locality as fig. 1. The erogenic forces locally shattered 

 the rock into a rubble. The broken fragments were cemented by secondary quartz, which 

 in the figure occnjiies as much space as the material of the original slate. In some places 

 the .slate fragments themselves are broken along two regular sets of planes inclined to each 

 other, which doubtless in each case represent shearing planes, both sets being produced 

 simultaneously, just as in the case of building stone crushed under the testing machine. 

 That these sets of planes do not intersect each other at right angles is doubtless largely 

 explained by the structure of the slate, which controlled to some extent the direction of 

 fracture and thus prevented the breaking from always occurring along the maximum shear- 

 ing planes. After the rock was brecciated and cemented as above described, a later movement 

 again slightly shattered it. The cracks thus formed, running through slate fragments and 

 matrix alike, arc filled with secondary silica. As lu fig. 1, the slate fragments are impreg- 

 nated with secondary hematite. Natural size. 

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