42 Dr. Mac CuUoch on the 



Both these classes of veins ramify, by subdivision, as they 

 proceed from the cefttral or principal masses ; but that feature 

 is most common in granite, while the veins of trap also differ 

 from them, very generally, in holding much longer courses 

 without any change of dimension. These differences, however, 

 do not destroy the analogy which subsists between these two 

 rocks ; and they admit of explanation by collateral circum- 

 stances which need not be examined in this place. 



The passage, both of granite and of trap veins, through 

 strata, is accompanied by peculiar appearances which are, in 

 both cases, of a similar nature, and which often, indeed, cor- 

 respond very accurately. In their immediate vicinity the strata 

 are displaced, distorted, or broken. In the case of trap also 

 fragments of the adjoining rocks are often entangled in the 

 vein; and if that occurs less frequently in granite veins, it 

 still happens sufficiently often, both in these and at the con- 

 tact of the larger masses of granite with the strata, to justify 

 that general analogy which is alone contended for in this place. 



The alterations in the mineral characters of the strata, which 

 occur at the junctions of these two classes of rock, are also 

 in both cases of a similar nature, resembling each other in their 

 general features, and only differing according to the previous 

 characters of the strata subjected to this influence. In both 

 instances of these junctions, the argillaceous schists are in- 

 durated and changed into siliceous schist. In some, the con- 

 tact of a granite vein converts that schist into hornblende, while 

 the contact of trap with the secondary argillaceous schist or 

 shale, frequently produces a substance scarcely differing from 

 basalt, and thus far analogous also to that mineral, which forms 

 the principal ingredient of this rock. The effects produced on 

 limestone, in both cases, correspond still more accurately 

 and obviously, because the limestones of the primary class, 

 which are those alone traversed by granite, differ less from 

 those of the secondary, which are principally subject to the 

 influence of trap, than any other of the analogous strata in 

 the primary and secondary classes do from each other. Such 

 limestones, when impure, or containing much siliceous and 



