44 Dr. Mac CuUoch on the 



thetical dogmas, will thus be directed to the use of their own 

 faculties in observing the phenomena which they may witness, 

 and to the exertion of their own judgments in reasoning from 

 them. 



Having thus pointed out the geological resemblances which 

 exist between the trap rocks and granite, it is necessary to 

 advert to one important point of difference, on which a greater 

 stress has been laid than the circumstances appear to justify. 



It has been remarked, that although the former are found 

 to lie above the secondary strata, which they chiefly accompany, 

 granite is never found in the same manner lying on the primary. 

 Hence it is argued, that, even if the igneous origin of trap be 

 admitted, the defect of this important feature in granite, is a 

 sufficient reason to refuse to it a similar origin. 



There are many collateral circumstances, however, to be con- 

 sidered, before the justness of this reasoning can be admitted. 

 The principal of these relates to the waste which the surface 

 of the earth has undergone ; but, on this subject, I need not 

 repeat that which is already familiar to geologists, and which 

 consists merely of general reasoning derived from analogies. 

 If indeed the instance quoted by Mr. Von Buch, in Norway, 

 of granite incumbent on conchiferous limestone, and the similar 

 fact immediately to be described as occurring in Sky, be ad- 

 mitted as examples of real granite, the doubt in question is 

 removed, and the fact of the super incumbence of that rock is 

 established. 



But it will be seen hereafter, that no advantage is taken of 

 these examples, as they are considered to be modifications of 

 the trap family. The term granite, here used in a strictly 

 geological sense, is limited to all the rocks of this character 

 which are subjacent to the primary strata only, or, when both 

 classes occur together, to both. The veins which proceed from 

 it also penetrate the primary strata only, and not the secondary, 

 and thus it is proved to be of a date prior to the deposition of 

 the latter. In this rigid view, therefore, of the meaning of 

 the term granite, if ever it is found in a superincumbent form, 



