454 GEOLOGY, 



THE PROPOSED QUANTITATIVE SYSTEM. 



The distinguishing characteristic of the more rigorous system designed to 

 meet the needs of scientific petrology is its quantitative chemical character. 

 All igneous rocks are classified primarily according to their chemical composition 

 and only secondarily according to their mineral constituents, texture, and other 

 characters. The rigorous application of the system requires chemical analyses 

 of the rocks, but as these are not available in many cases, the authors of the system 

 have devised a method of optical mineral analysis by which the nearly exact 

 proportions of all the constituent minerals can be determined, and by knowledge 

 of their chemical nature the results may be converted, by computation, into chem- 

 ical terms. This can only be done for holocfystalline rocks whose crystals are 

 large enough to be measured under the microscope, but aphanitic rocks may often 

 be approximately classified by comparison with similar rocks already accurately 

 determined. To facilitate this method of chemical analysis by measuring the 

 minerals, the chemical composition of certain common rock-making minerals is 

 expressed in proportional parts and tabulated, and is used somewhat as molecular 

 weight is in ordinary chemical analysis. Certain of these are selected as standard 

 minerals, the selection being such that the standard minerals embrace all the 

 essential elements that enter into the composition of rocks. All other minerals 

 are converted into their chemical equivalents in terms of these standard minerals 

 by the use of the tables. All the mineral constituents being thus reduced to 

 standard minerals, the classification is built up systematically on these standard 

 (or standardized) minerals. 



A new system of names is required, and these have been very skillfully formed 

 by selecting significant letters from the names of the leading minerals or from 

 words signifying their preponderance, so that short terms which carry their meaning 

 in their forms, are secured, and this has been done so that these are usually eupho- 

 nious, however strange they may seem to our preoccupied senses. For example, 

 minerals composed chiefly of silica and aZumina are called salic; those of ferro- 

 magnesian minerals, femic; those of aluminous /erromagnesian minerals, alferric, 

 etc. When in a combination of salic and femic minerals, the salic are extremely 

 abundant, the rock is persalic; if notably c?ominant, c?osalic; if the salic and 

 femic minerals are nearly equal, salfemic; if the femic are c?ominant, do femic; 

 if extremely abundant, perfemic, and so on, the system being mnemonic. This 

 method of deriving names is applicable only to a portion of the necessary 

 divisions. For the rest, a series of roots derived from geographic names, with a 

 system of terminations, has been employed. 



All standard minerals are divided into two groups of primary importance: 

 one of minerals characterized by alumina, as the feldspars, — orthoclase, albite, 

 anorthite, — ^leucite, nephelite, sodalite, noselite, and corundum, to which are added 

 the closely associated minerals, quartz and zircon. This is called the salic group. 

 The second group contains minerals characterized by iron and magnesia with 

 no alumina, as hypersthene (enstatite), acmite, olivine, magnetite, hematite, 

 and ilmenite, to which are added the closely associated minerals, titanite, perof- 

 skite, rutile, apatite, and all other rock-making minerals except those containing 



