IN GLEN TILT. 347 



masses of the strata, and such as have their adjacent sides more 

 nearly parallel. The gradation thus pointed out removes 

 any objection from the extreme case of difference in the form 

 of the vein, where the vein is in substance precisely similar to 

 the main rock, and visibly continuous without any abrupt tran- 

 sition. 



117. In the next place, we have the evidence of close analo- 

 gy, for extending the inference of a common origin to those 

 veins in the neighbourhood of the main rocks, which consist of 

 a sienite similar to that of the main rocks, but between which 

 and the main rocks no connection can be traced on the pre- 

 sent surface. These veins are in substance similar to the for- 

 mer, and they intersect similar strata in a similar manner. Their 

 proximity to the main rocks therefore argues that they either 

 are, or have been, connected with the main rocks, though that 

 connection does not now appear. 



118. With respect to all the veins which consist of a sienite 

 similar to that in the main rocks, an argument may be derived 

 from stating the question in a dilemma, between a common 

 origin and a different one ; and the probability of the former 

 must be proportionate to the improbability of the latter. Now 

 a sienite of these precise characters is by no means common, 

 either among minerals in general, or among those of the adja- 

 cent district ; and it must appear highly improbable, that two 

 separate processes should have agreed in producing it, with 

 such perfect similarity of substance, in these contiguous spots, 

 and there only. 



1 19. Among the hypotheses, by which a separate formation 

 has been ascribed to veins analogous in character, and situa- 

 tion, to those in Glen Tilt, is that of a cotemporaneous forma- 

 tion by secretion from the strata, while yet in a soft state. 

 Should this hypothesis be applied to those veins of sienite in 

 Glen Tilt, which are of a substance precisely similar to that of 



the 



