CLASSIFICATION IN LITHOLOGY. — GENERAL CONCLUSIONS. 55 



called by the same name. Again, if the mineralogical classification is adopted 

 it is found that the species of feldspar, which form the corner stone of that 

 classification, and their relations, have not been definitely settled ; and let 

 the method of determination adopted be what it will, they cannot be accu- 

 rately and surely distinguished from one another; therefore no system 

 should be placed on such an uncertain foundation. Such classifications cor- 

 respond to the Linnean artificial botanical classification, and hold about the 

 same relations to the natural classification of rocks as that does to the 

 natural classification of plants. The greater number of rocks separated by 

 these classifications into distinct species seem to be mere varietal forms of 

 certain natural species — the variation being due to alteration, or some 

 change in condition. Distinction should be made between superficial 

 weathering and the chemical and molecular changes that go on in all 

 eruptive rocks, after consolidation and exposure to the action of infiltrating 

 waters; that is, changes in the rock-mass as a whole — a change from an un- 

 stable to a more stable condition, a loss of energy. It is believed that to 

 these chanses is due in general the difference now observable between the 

 ancient and modern eruptive rocks of the same types. In other words, to 

 these changes is due nearly all, if not all, those distinctions which cause the 

 eruptive rocks to be divided into older and younger, or into pre-Tertiary, and 

 Tertiary and recent. It is, then, claimed that geological age has no bearing 

 on classification beyond this : that the older I'ocks of the same type or 

 species are, the greater is their alteration under like conditions ; and that 

 the greater number of the so-called rock-species of pre-Tertiary age are the 

 altered forms of rocks which were once identical with Tertiary and modern 

 rocks. The original or eruptive rocks of the universe appear to form a con- 

 tinuous series, from the most basic to the most acidic ; but for convenience 

 they are to be divided into definite species, t3q>es, or groups, which on the 

 boundaries will naturally fade into one another. The preponderance of 

 characters, and not the presence or absence of any one mineral ought to 

 decide the place of any rock in the s_ystem, and the original characters ought 

 to hold priority in classification over any that are of a secondary nature 

 (alteration, etc.). 



The natural classification, in its broader applications, can be employed in 

 the field as well as in the laboratory ; for, as a rule, all the characters of 

 rocks are so related to one another that from one the others can be inferred 

 with a fair degree of accuracy. 



