INTRODUCTORY. 6 



facts revealed by geological observation ; and how unsatisfactory general theories may 

 be when based upon the latter alone, is sufficiently exemplified by the fantastical 

 attempts made in all ages of geological science to interpret the geological structure of 

 the world from that of a limited region. The basis for argumentation can therefore 

 never be broad enough, and its enlargement should be, as it indeed is, one of the 

 chief objects of geological science. But it is not sufficient to content ourselves with 

 an accumulation of primary observations, which are in fact being infinitely increased 

 by the conjoint labors of geologists in all countries : it should be a higher object of the 

 student of geology, to compare the established results of observation, and to investi- 

 gate their mutual relations. The study of the structure of one mountain range, or of 

 several ranges comprised within a limited district, may lead to the establishment of an 

 elaborate theory of the mode of their formation, which may apparently answer per- 

 fectly well in that one case, but may be found inadmissible when generalized, even in 

 those cases where, by imperfect observation, one would expect to detect a great simi- 

 larity to the structure first observed. But in determining those features which are 

 common to a number of mountain ranges, or to certain orders of them which we may 

 discern among their generality, we may aspire to form conclusions which are more 

 generally applicable. It is particularly the auxiliary branches of geology to which 

 these remarks apply. The value of observations made in limited regions, or from a 

 limited point of view, on subjects such as the outlines of the morphological feat- 

 ures of the continents, the occurrence of mineral springs, the structure of mineral 

 veins, the age of those among them which carry a certain metal, the generality of vol- 

 canic phenomena, the mode of action of earthquakes, the nature of certain kinds of 

 rocks, and their part in the structure of the surface of the globe cannot be fully real- 

 ized unless the comparative method is applied in as wide a scope as we may be able to 

 do, and the mutual relations among the different modes of manifestation of force, or 

 among the properties of the kinds of matter upon which it acts, or the bearing of all 

 these relations to each other and to the evolution of the globe, are investigated from 

 as many points of view as we may detect, and in as many combinations as possible. 

 We may then be able to gain a foundation for argumentation on more involved prob- 

 lems, consisting not of imperfect premises, nor of a confused accumulation of facts, 

 but of established truths of a higher order. 



The mode of origin of the non-foliated crystalline rocks, made up of silicates,* 

 is among those subjects which have at all times, but at no time more than of late, 

 commanded a great deal of attention, and given rise to the establishment of numerous 

 theories, each of which was applied in a general way, if not by its author, then by his 

 followers. It is well known how conflicting they apparently are, and what weighty ar- 

 guments have been brought in favor of as well as against each of them. The only 

 method, promising success, of weighing the merits of these different theories, or of 

 modifying them in accordance with the general advance of science, appears to be, to 



* I have for these applied the name " eruptive rocks " in the following pages, considering that, wherever we have 

 occasion to observe them, they are not at their original seat, but ejected from it towards the surface. The reasons supporting 

 this position will be more fully mentioned in the chapter on the origin of volcanic rocks. 



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