164 



LOWER PENINSULA. 



200 feet wide, succeeding to which arc other slates of the inferior 

 unmarketable quality. All these strata are conformable in strike 

 and dip. Near the bay the horizontal sand-rock ledges are again 

 encountered. 



In the quarry of the Huron Bay Slate Company, the slate dips 

 south, at an angle of from 45° to 60°. Three cleavage plains in- 

 tersect the slate and divide it into rhombical blocks. The princi- 

 pal cleavage, with which alone it can be split into roofing slabs, is 

 identical with the plane of stratification. 



By another cleavage, intersecting the slates in horizontal direc- 

 tion, the walls of the quarry appear as if composed of a sequence of 

 thick horizontal beds. 



The rock mass here is frequently cleft by narrow, vertical fissures, 

 which are filled out with different mineral substances essentially 

 distinct in genetic character. 



• One of these fissures seen in the walls of the quarry, of irregu- 

 lar, somewhat tortuous form, in some places wide, in others nar- 

 row, and more than once divided into two branches, which subse- 

 quently unite, is filled with a compact, dark, greenish-colored 

 rock mass, of amorphous or finely crystalline fracture, and inter- 

 spersed with acicular sparry crystals, together with granular crys- 



