” 
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 Cat’s Nick. 
I po 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 greenftone, till Sir James Hart 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 rubbifh, 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- 
fible. Nothing, however, of much intereft, was by this means 
difcovered. The dike, after bending a little to one fide, conti- 
nued its courfe downwards. . 
‘Tue {pace which this dike occupies, may be from fix to eight 
feet wide ; its width varies a little in fome parts, and thefe va- 
riations are apparently increafed, if the feGtion 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 rufty- 
3G 2 brown 
ly underftand by mineral veins. The firit are formed of one uniform rock, 
compofed in all their parts. of the fame conftituents, 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 
. Ipar, 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 mafles; therefore, in ufing the term vein, when applied to greentftone, 
granite, or the like, it muft be underftood as a generic term, of which these lat. 
ter, {pecify the variety. 
