IN GLEN TILT, 35'J 



tions of the adjacent limestone, inlroduced into crevices in the 

 gneiss, or by secretion from the strata themselves, in which 

 carbonate of lime is frequently an ingredient, and limestone 

 an interstratified substance. Sir James Hall observed that 

 carbonate of lime, when fused under compression, is a viscid 

 substance. In some limestone strata not far from the sienite, 

 on the eastern side of the bridge, (par. 89.) there are rapid in- 

 curvations, without fracture, which strongly suggest the idea 

 of a viscid state in the rock, at the time of their formation. 



132. It has been mentioned, that there are shifts in some of 

 the veins *. Whatever theory be adopted respecting the man- 

 ner in which the sienite, and the stratified masses, assumed 

 their present relative position ; the shifts prove that, after they 

 had done so, there was a dislocation among these rocks, in 

 which the stratified masses, and the sienite between them, were 

 moved together. But the fact that these rocks are now com- 

 pact, and without cavities, can hardly be reconciled with the 

 idea of such dislocation ; unless by admitting that, when it took 

 place, a certain degree of softness pervaded their whole sub- 

 stance, both the strata, and the sienite. It accords with an ig- 

 neous theory to explain these circumstances by supposing, that 

 the dislocation occurred not long after the introduction of the 

 sienite, while the heat continued sufficient to maintain a softness 

 through the whole. But the difficulty of conceiving such a soft- 



VoL. VII. P. 11. Z z ness 



* In a rock near the bridge, there was noticed (par. 84.) a shift in one 

 •f two veins, which are apparently of posterior formation to the sienite. It maj 

 be worth while to observe, that these veins dividing both the sienite, and 

 the imbedded mass of strata, can hardly be ascribed to secretion ; for it is highly 

 improbable, that the strata, whicii are of granular quartz and hornblende-slate, 

 should have agreed with the sienite in secreting the same substance ; nor is it 

 likely, that a secretion from the one, should have filled that part of the crevice, 

 which had been formed in the other. 



