8 R1CUTII0FEN THE NATURAL SYSTEM 



not of one only but of various principles, compares and weighs the results obtained by 

 each of them, and accepts them as final only when perfectly harmonizing among each 

 other. It is then that it tries to determine what principles are most available for 

 establishing the higher orders, and which for the subdivisions. The singular complica- 

 tion which is peculiar to the classification of rocks, is, besides the reasons already 

 mentioned, due in a great measure to the fact that geology combines the double func- 

 tions of a historical and an inductive science, while in petrology we have besides the 

 requirements of a descriptive natural science. The natural system of rocks should 

 therefore be based, not only upon the entire range of their petrographical characters, 

 such as mineral composition, chemical composition, texture, and specific gravity, but 

 also upon their mode of origin and geological occurrence. Classification of objects 

 and classification of relations are, with them, closely connected, and should be made 

 to assist each other. 



The question may be raised, whether a natural system of rocks based upon 

 such principles can be established at all, and if it can, whether it would be of any use 

 for the advancement of science. To the first question, the answer must be in the 

 negative, as far as sedimentary rocks are concerned. They have been formed by a 

 complexity of circumstances, and just so complex and infinite in variety are they, in 

 respect to chemical and mineral composition and all external characters. To analyze 

 in detail their mode of origin, and the sources from which their material has been 

 derived, transcends the faculty of human intellect, and it would be a hopeless task to 

 attempt to discover any laws regulating the boundless differences of their composition. 

 They are thus debarred from natural classification, though its principles may be applied 

 imperfectly to the establishment of some general groups. "We arrive at similar con- 

 clusions in regard to those rocks, the sedimentary origin and subsequent metamorphism 

 of which can be proved. Accidental and local circumstances have played as conspic- 

 uous a part in their first deposition as was the case in regard to those sedimentary 

 rocks to which the term metamorphic has not been usually applied. But as meta- 

 morphic processes of a certain nature have ordinarily affected extensive tracts of 

 these rocks, and similarly pervaded great thicknesses of them, the local differences of 

 their action having been apparently more in degree than in mode, they have occa- 

 sioned a certain similarity of effect which partly conceals the original differences in 

 the composition of the rocks affected ; and it appears that the differences in the kind 

 and intensity of metamorphic action, though recognizable only in their final results, 

 will, when better known, afford a convenient principle for a classification which may 

 have some similarity with, but not the full requirements of, the natural system. Tt is 

 different with those rocks which on the surface of the globe appear as intrusive or 

 eruptive masses. Notwithstanding their infinite variety in character and composition, 

 they are connected by definite relations which bring their elementary composition 

 even within range of mathematical calculation. Their recurrence in the most widely 

 separated countries, with similar external character, identical chemical composition, 

 and in analogous relative order of succession, is another distinguishing feature of 

 eruptive rocks. For these reasons, as well as in virtue of other peculiar characters 



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