50 Dr. Mac CuUoch on the 



rarely in the other. It is sufficient to have indicated some facts, 

 hitherto unknown, which add a mineral ogical resemblance to 

 those formerly acknowledged to exist between two classes of 

 rock so remote in origin, and to have given a brief sketch of 

 the other circumstances of analogy which were required to 

 illustrate the main object for which these facts have been 

 brought forward. Those who may hereafter examine this 

 question as a matter of geological theory, and as connected 

 with the causes which have influenced the dispositions and the 

 characters of the rocks that constitute the visible portion of the 

 earth, will thus be furnished with additional facts from which 

 to reason. 



But if, in the present state of geological science, the collection 

 of facts is necessary, it is not the less incumbent on the observer, 

 to reflect on the main object to which all such facts are des- 

 tined, and to keep steadily in his view the great purposes of all 

 such investigations, namely, the establishment of analogies, 

 and the discovery of those causes, a knowledge of which is no 

 less useful as a guide to our inquiries, and as forming an in- 

 dispensable part of the science, than it is an invincible desire 

 in all inquiring minds. In concluding this communication, 

 therefore, I shall, in the briefest possible manner suggest those 

 reasonings which the facts in question appear to indicate. 



The arguments by which the igneous origin of the trap rocks 

 is supported, are so well known that they do not require to 

 be repeated ; were it even one of the objects of this paper to 

 enter on the general merits of this question. That doctrine is 

 now indeed so universally received among all those who have 

 shaken off the bondage of authority, and who have both the 

 capacity and the inclination to observe and to reason for them- 

 selves, that it may be considered as established. But the 

 phenomena displayed by granite, although, in the most essential 

 points, resembling those which occur in the trap family, have 

 as yet failed to produce the same general conviction with regard 

 to the igneous origin of that rock. Into the arguments of a 

 geological nature by which this doctrine may be supported, I 



