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344 ON THE HIGHLANDS. 



This rock, as formerly observed, contains in 

 general crystals of felspar. It occurs, however, 

 in different places, without the porphyritic struc- 

 ture : and has then a strong resemblance in some 

 instances to basalt, but more frequently to clink- 

 stone ; as at the top, where, if struck with a ham- 

 mer, many of the fragments or loose pieces, ring 

 like metal. Its fracture exhibits the varieties of 

 the splintery, the flat conchoidal, and the foliated ; 

 and its colours, those of the greyish-black, and 

 the dark-greenish-grey. But it does not appear 

 to be ever vesicular, or translucent on the edges ; 

 it contains no traces of olivine or augite, so far 

 as my observation extended ; nor is the principal 

 fracture slaty in the great, a character without 

 which the porphyritic varieties cannot be consider- 

 ed as porphyry-slate. 



True clinkstone and basalt, indeed, are sub- 

 stances which belong to a newer and very dif- 

 ferent aera of formation, and do not seem to be so 

 purely chemical in their nature as the rock we 

 are considering. But the affinity of external 

 characters now pointed out; illustrates one of the 

 principles of the Geognosy and appears strongly 

 to support a conjecture of my friend Professor 



* " The resemblance of the newer porphyry to the 

 f newest flcetz> trap formation, is deserving of attention. 

 " The points of agreement are, in the stone itself \ in the 

 4£ structure of the rocks," &c» Jameson's Geognosy, 

 p. 138. 



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