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SILICEOUS SCHIST. 



The large columnar structure formerly mentioned as 

 occurring at Dunbar, is very instructive ; because the 

 original beds consist of shale and sandstone, and the 

 different jaspers which result from the induration of 

 both, remain as distinguishable as the original sub- 

 stances. 



I know not how to frame a satisfactory definition of 

 jasper; but, in general, it may be conceived as an ex- 

 tremely indurated clay, of which certain varieties ap- 

 proximate to hard pottery, and others to porcelain. 

 Having no predominant texture, it is equally frangible 

 in every direction ; and though the fracture is gene- 

 rally minutely granular, and the surface arid, it is oc- 

 casionally splintery, or even conchoidal ; while the 

 minuter fragments may even be translucent. It is not 

 easily confounded with any rocks, except the indurated 

 claystones or the pitchstones. From the intimate na- 

 ture of their connexion, it cannot always be distin- 

 guished from the former. From the latter, it is more 

 easily discriminated, by the absence of the resinous 

 lustre and peculiar transparency of the fragments ; 

 although pitchstone sometimes approximates to it in 

 aspect. If Nature has not created definite boundaries, 

 it is in vain for mineralogists to attempt it. For the 

 varieties, I must refer to the Classification of Rocks. 



Siliceous Schist. 



So thoroughly has this rock been misapprehended 

 by geological writers, that I must, as usual, limit 

 these remarks on it to my own observations. That 

 "flinty slate," as it is called, can have no claim to be a 

 leading rock in the system, as has been stated, will be 

 immediately seen. It occupies two distinct situations ; 

 the one among the primary, and the other among the 

 secondary strata ; appearing, in both, to be an acci- 



