IN GLEN TILT. 355 



substances, agree very well with an igneous theory. Sir James 

 Hall found carbonate of lime, when fused under compression, 

 to be a powerful solvent of silex. This offers an explana- 

 tion of the gradation on the sides of the veins of felspar 

 in the strata of limestone at B, and of some similar ap- 

 pearances at D, (parag. 95.) ; also of the siliceous character in 

 the mass of limestone adjacent to the sienite at D, below the 

 bridge, (Parag. 81.). In these cases, it may be supposed that 

 the carbonate of lime, when fused by heat, derived a portion 

 of silex from the felspar in contact. The siliceous character of 

 the vein of granular limestone, or compact dolomite, at C, may 

 be accounted for in the same manner. On the other hand, it 

 may be supposed that the sienite of the main rocks derived 

 its small portion of carbonate of lime by solution from the ad- 

 jacent strata. But, whether we adopt an igneous theory, or 

 any other, there are facts that it will appear difficult to ex- 

 plain ; for we have seen that there are masses of hornblende- 

 slate, and other substances, which, though penetrated by the 

 felspar of the sienite, have still preserved a stratified texture, 

 and the sharpness of their angles ; and it is not easy to con- 

 ceive a state of aggregation soft, or porous, enough for the for- 

 mer circumstance, and at the same time firm enough for the 

 latter, or what agent could have produced it *, I have heard 



Sir 



* A remarkable example of fragments of a rock penetrated by an extraneous 

 substance is mentioned by M. Daubuisson, as having fallen under his own obser- 

 vation. In a note to § 72. of his translation of Werner's Theory of Veins, he 

 tells us, that near Freyberg a metalliferous vein was found divided into several 

 small veins, containing between them fragments of gneiss, through which g.de- 

 na was disseminated in so large a quantity, that the quintal ©f gheiss contained, 

 thirty pounds of lead. He adds, as something more remarkable, that it was, 

 only the gneiss of the vein, which contained galena ; for the gneiss of the rock, 

 even where adjacent to the vein, contained none at all. 



