SCHEMES OF CLASSIFICATION — THEIR RELATIVE VALUE. 33 



Section V. — Classification hascd on Mineral Composition. 



If the artificial schemes of lithological classification are examined, it 

 will be found that they are generally based on the mineral composition, 

 the geological age, and the structure of the rocks, some rocks even being 

 defined by a statement of that which they are not, or that which they do 

 not contain. 



It does not accord with the limits of this paper to enter upon any thor- 

 ough critical discussion of the application of such principles, but a certain 

 examination of them may be made, so far as they bear on the method of 

 classification it is proposed here to use. Amongst the minerals of which 

 the chief use is made in classification there may be mentioned the feld- 

 spars, including leucite and nephelite, also olivine, quartz, the micas, pj^rox- 

 enes, and the amphiboles; although any mineral is liable to assume in 

 special cases sufficient importance in these artificial schemes to found specific 

 distinctions upon. 



Of these minerals the most important are the feldspars, and on their 

 presence or absence, and on the species or type of feldspar present are 

 founded some of the most important divisions of the rocks. In order to 

 successfully use any mineral in classification it is necessary that it should 

 be a determinate quantity — that is, it should alwaj-s have in the rock one 

 mode of formation only — that its specific limits shall be well marked, and 

 that it shall be accurately determinable with ftiir fiicility. 



The Feldspars. 



That the feldspars originate in all three of the methods given previously 

 for the origin of rock minerals — foreign, indigenous, and secondary — the 

 writer thinks cannot be denied ; although, for the most part, they appear 

 to be indigenous. In classification of this kind, the most important question 

 about the feldspars is, what are their divisions and the diagnostic characters 

 of them. A sketch of the various prominent opinions regarding their 

 constitution will best answer our question. 



In 1846 Scheerer held that the feldspars were different grades of satura- 

 tion, of a radical compounded of equal atoms of R. and AL* Later he 

 remarked f that it was permitted to regard all known feldspai's as chemical 



* Ami. Physik Clicniic, 1846, kvLii. 337; Am. Jour. Sci. ISiS (2), vi. 61. 

 f Ann. Physik Cliemie, Ixxxis. 19. 



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