MODE II. CLAY SLATE. 5255 



means shale incumbent on coal; but from 4 to 

 8 of iron may be considered as commonly be- 

 longing to clay slate, while the siderous or com- 

 mon slate, eminently so called, contains from 10 

 to 20. 



Ferber's primitive slate is argillaceous, with 

 particles of mica, and crossed by veins of quartz, 

 which more rarely happens in the siderous 

 kind ; and it often appears in undulating strata. 

 He adds, that in the Vicentine and Veronese 

 territories it is regarded as the deepest rock, any 

 subjacent granite not having been discovered. 

 It contains as usual metallic veins, which often 

 run between it and the incumbent lime-stone*. 

 Patrin has little enlarged on clay slate, though 

 a rock of the first importance; but indulges his 

 imagination, that the vast beds of clay have 

 been produced by muddy eruptions of sub- 

 marine volcanoes. 



The fine stone used for sharpening razors, Hone, 

 called a hone, is commonly a clay-slate, con- 

 taining, like the others, about 60 parts of fine 

 silex. It is often of the cameo kind, or disposed 

 in layers of different colours, the upper of a 

 whitish yellow, and the under of a reddish grey; 



* Italy, 37- Da Costa, p. 165, says, the black slate of Claris, in 

 Swisserland, which rises in slabs, contains impressions of plants 

 and fish ; and is of course secondary. 



