100 GRANITE. 



brought to their present places in a state of fusion, 

 and that every case of contact with the strata must he 

 one of this nature. But the older must have been 

 elevated by the later in a solid state, as the strata have 

 been, whatever changes heat may have induced in 

 them at the contact of the fused mass, as it has in 

 gneiss ; so that some of the disputed appearances 

 respecting granite may he explained on this ground. 

 On the often discussed overflowing of granite, I must 

 remark, that the facts here described, and our know- 

 ledge of the changes of crystallization which fused 

 rocks undergo in consequence of more or less rapid 

 cooling; prove that an overflowing granite might be a 

 trap in every point of character. Whence the present 

 most important, and new conclusion in geological 

 philosophy, namely, that as agents in the changes of 

 the earth, granite and trap are identical, and that 

 while there is an uninterrupted succession of these, 

 the fact that their characters are interchangeable, 

 even when of one sera or in one mass, shows that geo- 

 logists have drawn a boundary where there is none, 

 and that they cannot even be sure of any one trap, 

 except under the clear proof of recent intrusion ; the 

 only test of date, yet never a negative one. The 

 explanation and general bearing of some minor points 

 are so obvious, that the reader ought to be indignant 

 at seeing them here stated. 



Granite presents great variety in its tendency to , 

 decomposition and in the rapidity of that process. As 

 formerly noticed, it does not always require exposure 

 to air; since large masses are occasionally found 

 destroyed, deep within the earth. If invariable rules 

 cannot be estahlishcd, those which contain red felspar 

 are least exposed to this change ; from ohvious che- 

 mical causes ; though the rnica sometimes produces 



