APPENDIX A. 



165 



tals of pyrites and some leaflets of a shining brown mica. This 

 mass is most intimately united with the slate wall on both 

 sides, and has all the appearance of an injected igneous body. 

 Near the surface the fissure is wider, and the inclosed rock sub- 

 stance spreads laterally over the immediately surrounding surface. 

 The greatest width of the vein is perhaps a foot, but it shrinks in 

 places to not more than half an inch. 



Another class of fissures, which are of more frequent occurrence, 

 are filled with milk-white quartz, mixed with calcspar and iron 

 pyrites crystals. These are rarely over an inch wide, and the mat- 

 ter filling them is evidently deposited there from aqueous solutions 

 percolating through the fissures. In some of them, the walls are 

 straight and smooth ; in others the slate has broken through in zig- 

 zag lines, with strongly indented, saw-like edges, often slightly dis- 

 located so as to bring the teeth and indentations in opposition. 



The process of quarrying the slate is attended with great waste, 

 which is to some extent unavoidable. The slate is often unfavora- 

 bly cleft, and the uplifted layers are sometimes curved or warped 

 to such a degree as to make them unfit for roofing ; but, besides 



