Ch. XL] THE PRIMARY ROCKS. 227 



formed from materials deposited and arranged by water ; the 

 other, by the action of an internal fire, at considerable depths 

 below the surface. These conclusions are said to be founded 

 on physical evidence, on facts recorded by numerous ob- 

 servers in various parts of the world : and it may therefore 

 appear to be an idle waste of time, an attempt of no little 

 presumption, to make even a show of assaulting a position so 

 strongly fortified. There is no intention of disputing the 

 correctness of the facts, when such have been sufficiently 

 investigated and faithfully described ; but the evidence which 

 these afford, may be sometimes disputed : for what is this 

 physical evidence of which we so often hear, and to which 

 theorists so frequently appeal ? It is only a testimony recorded 

 in hieroglyphics of an unknown character, and which may 

 therefore admit of divers interpretations. In our attempts to 

 decipher these characters, no solution can be admissible, 

 unless it be applicable to all without exception. Now, the 

 prevailing theory satisfactorily explains a great body of these 

 facts, but it will be the object of the following pages to show 

 that there are some phenomena which it does not appear to 

 interpret in a clear and convincing manner. For instance, 

 we are taught that gneiss, and other primary strata, which so 

 nearly resemble the granitic rocks, especially at! their j unction 

 with each other, are merely sedimentary deposits altered by 

 the contact of granite in a state of fusion ; and, that the con- 

 dition of secondary strata next trap rocks, clearly indicates 

 that such changes do take place under similar circumstances : 

 but, after making every allowance for the comparison of small 

 things with great, we shall strive to show that the cases com- 

 pared are not analogous ; that the evidence brought forward 

 bears witness to changes produced by heat, but not to such 

 changes as the primary strata are supposed to have under- 

 gone ; viz. an assimilation of the aqueous to the igneous rocks, 

 by the introduction of additional elements into the composition, 

 of the former. 



Lastly, the prevailing theory embraces igneous convulsions, 



Q 2 



