IN GLEN TILT. 345 



90, 94.) The principal exceptions are, where the strata are of 

 gneiss, or of granular limestone *. 



113. Shifts occur in the veins of sienite, and its gradations. 

 (Parag. 62, 84, 98.) 



114. These appearances afford grounds for some inferences 

 respecting what took place in the formation of these rocks. 



115. A crystalline character in a rock is generally admitted 

 to prove, that it was once in a state to a certain degree fluid. 

 An opposite notion has indeed been advanced by Dolomieu, 

 and Mr. Gregory Watt ; but, both the mineralogical appear- 

 ances, which suggested this opinion to Dolomieu, and the ex- 

 perimental results, which led Mr. Watt to it, admit of an 

 easy explanation, from what Sir James Hall ascertained by 

 experiment, with regard to the varieties of structure produced 

 by cooling under different circumstances ; and no sound ar- 

 gument against the common opinion can be drawn from 

 what either of them has described. We may therefore in- 

 fer, that the main rocks of sienite have been in a state 

 to a certain degree fluid; whether by aqueous solution, 

 or igneous fusion, I do not now consider. We may make 

 a similar inference for the sienite, and the gradations from 

 it, which now occupy the intervals between the separate 

 masses of the strata, often assuming the form of veins. 

 I would farther infer, that the fluid from which the main 

 rocks of sienite crystallised, and the fluid from which the 

 veins of sienite, or its gradations, crystallised, were one and the 



same; 



* The least real gradation may in appearance be a considerable one, where 

 the surface of the rock cuts the plane of the junction at a small angle. This is a 

 source ^of deception to be guarded against in observation. 



