Dr. Mac Culloch on Jasper. 71 



and yellows are the most remarkable ; but it also occurs of 

 grey, brown, purplish, and greenish tints. 



B. More indurated, and resejnhling the base of certain por- 



phyries. 



a. Simple, and of one colour, green, red, brown, yellow, 



or even black. 



As this substance is generally collected for the sake of its 

 colour, the more decided tints only are commonly found in 

 cabinets, but it occurs of various hues.' 



b. Striped with different colours, in consequence of a laminar 



structure. 



The Siberian green and red variety belongs to this ; it also 

 occurs of different tints of red alternating, or of greys, or of 

 other colours. The latter are also enumerated among the 

 siliceous schists. 



c. Spotted or variously mottled, in consequence of a con- 



cretionary spheroidal structure. 



The Siberian spotted jasper ranks under this variety. The 

 most common colours are, reddish and pale ochre, obscure red 

 and white, and brown and ochre. 



C. Highly indurated, with an aspect approaching to that of 



chert, or even to agate ; into which it passes, as it does into 

 chert and quartz. 



a. With a somewhat granular fracture. 



b. With a granular splintery fracture. 



c. With a splintery fracture passing to the conchoidal. 



d. With a flat fracture passing to the larger conchoidal. 



The two latter varieties are among the most esteemed, as as- 

 suming the best polish. The colours most prevalent are reds 

 and yellows, simple or intermixed in various ways. The varieties 

 of C occur chiefly among the primary rocks. 



D. Intermixed in various ways with chalcedony cither white or 



coloured, and apparently at times pasaing into that sw6- 

 Uance : jasper-agate of lapidaries. 



