VICINITY of EDINBURGH. 419 



which the cart-road, along the bafe of the rock extends, a 

 few feet beyond a gap, known by the name of the CaVs Nick. 



I do not think this vein attracted the attention of geologifts 

 in any particular manner, prior to 1805. It certainly was ob- 

 ferved long before that period, but was not known to extend 

 through the bed of greenflone, till Sir James Hall and myfelf 

 • noticed, that after cutting the fandftone, it continued its courfe 

 uninterrupted to the top. This obfervation contributed very 

 much to increafe our curiofity, and a man was employed to 

 clear away the foil and rubbiih, which had accumulated on the 

 furface. A confiderable portion of the rock was foon laid 

 open, below the point from which it was at that time vi- 

 iible. Nothing, however, of much intereft, was by this means 

 difcovered. The dike, after bending a little to one fide, conti- 

 nued its courfe downwards. 



The fpace which this dike occupies, may be from fix to eight 

 feet wide ; its width varies a little in fomc parts, and thefe va- 

 riations are apparently increafed, if the feclion which is obfer- 

 ved be not at right angles with the walls. That portion 

 embraced by the ftrata, which we found principally co- 

 vered with debris, was very much decompofed, prefenting on 

 the furface a certain degree of nodular exfoliation, of a rufly- 



3 G 2 brown 



ly underftand by mineral reins. The firft are formed of one uniform rock, 

 compofed in all their parts of the fame conilituents, and differing only in po- 

 fition, from the beds thefe materials more ufually form ; while the latter, 

 though fometimes formed only of one fubftance, fuch as quartz or calcareous 

 fpar, are generally compofed of a feries of foffils, arranged in lines parallel to 

 the walls. No fuch appearance ever prevails in rock veins, or conftituting 

 mountain maffes ; therefore, in ufing the term vein, when applied to greenflone, 

 granite, or the like, it muft be underftood as a generic term, of which these lat- 

 ;er, fpecify the variety, 



