232 ON GRANITE. " 



fragments of fchiftus, often contained, and completely infu- 

 lated in thefe veins. Hqw thefe fragments were introduced 

 into the fifiures of the fchiftus, and fuftained till they were 

 furrounded by the matter depofited by water, is very hard to 

 be conceived ; but if they were carried in by the melted gra- 

 nite, nothing is more eafily underftood." To this objection, 



It is anfwered, it is only neceflfary to anfwer, that the apparent infulation of. 



infulated. arC n ° ' ma ff es °? ^ one & veins, is occasioned by our only feeing part 

 of the mafs ; for when the whole is feen, we invariably find, 

 that it refts upon another fragment, or upon the fides of the 

 veins. At p. 31 4, when defcribing the junction of the granite 

 with the other primitive ftrata in the ifland of Arran, Profeflbr 



And with regard Playfair remarks, " Along this line, particularly on the fouth, 



tot e granitic wnerever the rock is laid bare, and cut into by the torrents, 

 veins or nume- . . . 



jousmaflesin innumerable veins of granite are to be feen entering into the 



"n hlf f U A n ^ fchiftus, growing narrower as they advance into it ; and being 



directed in very many cafes, from below upwards, they are 



precifely of the kind which the infiltration of water could not 



produce, even were that fluid capable of diflblving the fub- 



ftances which the vein confifts of. From the fouth face of 



this mountain, and from the bed of a torrent which interfe&s. 



it very deeply, Dr. Hutton brought a block of fchiftus, of fe- 



veral hundred weight, curioufly penetrated by granite veins, 



including in them many infulated fragments of the fchiftus." 



it is remarked In the iiland of Arran I have had frequent opportunities of 



that they are examining thefe kind of veins; they are generally found in 



and have other gneifs, are very much incorporated with it, and often both 



figns of being extremes of the vein are to be obferved in the fame bed. 



nearly coeval ' . . . . ' , _ , ' , 



with the gneifs. From this it is evident that thele cannot be conndered as gra- 

 nite veins, fince they are nearly of coeval formation with the 

 gneifs, and have no communication with any formation, older 

 or newer than that in which they occur *. Werner remarks, 

 that veins which are nearly coeval with the vein rock, are 



* Although we find in gneifs veins filled, and layers compofed 

 of a rock which, ory&ognoftically confidered, has every character of 

 granite, yet the geognoft juftly views them as varieties cf gneifs. 

 It often happens in a mountain of gneifs, that ftrata occur which 

 are not to be diftinguifhed from mica flate; thefe, however, are 

 merely accidental, and the whole is therefore to be referred to 

 gneifs. This admirable mode of inveftigation, which was firft dif- 

 covered by Werner, has been of the greateft utility in geognofia. 



very 



