SILICEOUS SCHIST. 563 



colours are sometimes also interl am mated. They 

 are frequently also interlam mated with chert or 

 with granular limestone, for reasons already des- 

 cribed. 



D. Extremely brittle, hard, and shining; the 

 fracture large or small conchoidal, and the fragments 

 sharp and cutting-. 



l C5 



This is the Lydian stone of mineralogical 

 writers, and is almost always of a pure black. 



E. With an internal spheroidal structure, pro- 

 ducing a botryoidal surface on weathering. 



This variety differs in hardness and in colour ; 

 the latter is black or grey. 



These five varieties, being derived from the 

 shales which accompany the coal strata, occasion- 

 ally contain minute shells, sometimes compressed 

 and deformed, as already mentioned. 



F. Laminar, with alternate colours, and forming 

 some varieties of the striped jasper of mineralogists. 

 The colours are commonly shades of red, brown, yellow, 

 and purplish black, and these kinds appear to be de- 

 rived from the coloured shales. 



G. Containing imbedded crystals of quartz, and 

 of a porphyritic aspect. 



All these substances, as might be expected, 

 and as already noticed, pass into the different 

 rocks from which they are derived. 



o o 2 



