IN GLEN TILT. 37 1 



feet. This porphyry has for its base a brownish-red compact felspar, 

 which is well characterised, and contains crystals of quartz thinly in- 

 terspersed. Its red colour makes it very conspicuous on the banks of 

 the river, and the outgoing of the bed may be traced on both sides in 

 the slopes of the mountains. 



Note B, Parag. 120. 



The minuteness, and intricate reticulation, of the smaller veins, 

 may be urged in support of the hypothesis of secretion. In these 

 points indeed they resemble those veins of quartz, and of calcareous 

 spar, which are often found running in all directions through various 

 stratified substances, and which it is difficult to conceive any other 

 process, but secretion, to have produced. However, after having re- 

 jected the hypothesis of secretion for the larger veins of sienite, it 

 would be absurd to have recourse to it for the origin of small veins, 

 which are evident ramifications, and gradations, from them. 



The gradation on the sides of the veins in some places may also be 

 insisted on, as favouring the hypothesis of cotemporaneous formation 

 by secretion. But distinctness, or gradation, on the sides of veins, are 

 equivocal circumstances in a question about their formation That 

 distinctness will not prove posterior formation, is evinced by its oc- 

 curring in veins, which there is every other reason to suppose secret- 

 ed; as in the minutely ramified veins of quartz running through 

 clay-slate ; in the small veins of calcareous spar in limestone ; in the 

 veins of calcareous spar, which cross dykes of whinstone, and do not 

 penetrate their walls * ; also in the lenticular veins of brown spar, or 



spathose 



* A good example of this occurs in the dyke that crosses the Water of Leith, 

 sbove St Bernard's Well. 



