42 Dr. Mac Culloch on the Origin^ Materials, 



that very striking analogy, now at last so well understood, in 

 which beds of shale beneath trap are actually converted into 

 Lydian stone ; a substance differing from it, almost solely in the 

 compactness and uniformity of its texture. 



We thus lastly arrive at gneiss ; a rock which often bears the 

 ftiarks of igneous consolidation in a still greater degree than those 

 of aqueous deposition, but in which it is almost unquestionable 

 that both have been combined. Where gneiss is at a distance 

 from granite, its laminar and stratified disposition is most perfect ; 

 where in its vicinity, it is most obscure ; indeed, so obscure, as 

 At length to disappear. This is precisely what might be expected 

 to happen on this view of it^ double origin ; namely, the application 

 of heat in unequal degrees, to a series of beds deposited from 

 water, and, probably, like quartz rock, originally consolidated 

 from it also. Where it is most remote from granite, although its 

 mineral materials should be the same, they are disposed in a dif- 

 ferent manner; or are more rigidly laminar and more inde- 

 pendent. Where it is most immediately in the vicinity of that 

 rock, and more particularly when it abounds in granite veins, the 

 structure becomes analogous to that of granite, or to one in which 

 there is that mutual penetration of ciystals which can only take 

 place in a fluid of fusion. At length it actually passes into the 

 contiguous granite ; losing that parallelism of the parts, and those 

 llist remains of the laminar disposition, which had gradually been 

 decreasing. 



' It is by no means difficult to imagine this combination of causes 

 and of effects ; a state of softening or semifusion, sufficient to 

 allow the integrant parts of a stratified watery deposit to enter 

 into new combinations, and to crystallize without the loss of the 

 Original marks of stratification. These, indeed, are often pre- 

 served in gneiss, by the alternate interposition of beds and laminae 

 of hornblende, and by that only ; just as in the watery joint 

 deposit of sandstone and shale, the latter substance is often the 

 only indication of stratification that can be procured. 



That such re-crystallization ean take place in a rock which is 

 heated to a point short of actual fluidity, is proved by Mr. Watt's 



