296 IDDINGS— PROBLEMS IN PETROLOGY. [April 21, 



petrographical facts involved in a comprehensive description of 

 igneous rocks. 



Recognizing the existence of continuous series of petrographical 

 factors, chemical, mineral and textural, necessary to the complete 

 description and definition of igneous rocks, the problem presents itself 

 of dividing the complex series of rocks so characterized into parts 

 that may be described in a comprehensive and systematic manner. 



A familiar example of a physical series divided in a regular man- 

 ner for purposes of exact use is that of temperature, partitioned in 

 degrees of definite proportions of a continuous scale. It is undoubt- 

 edly an arbitrary method and differs distinctly in three commonly 

 employed usages. It might be a more "natural" method to express 

 temperature with reference to the melting points of a series of sub- 

 stances ; and the value of certain of these definite points as datum 

 points is well known. But the merits of the arbitrarily, but very 

 naturally, divided scale are attested by its universal employment. 



The proposal to partition the petrographical series into quantita- 

 tively definite parts, as has been done in the Quantitative System of 

 Classification of Igneous Rocks, the size of the divisions being arbi- 

 trarily chosen, has excited criticism by some petrographers, who 

 consider. it arbitrary, artificial and not " natural." But the objection, 

 that measured precision condemns a classification of igneous rocks, 

 because it makes evident " its aloofness from the scheme of nature 

 based not on arithmetical but on physical and chemical principles,"^ 

 suggests a lack of appreciation of the mathematical precision of 

 stoichiometric chemistry, and a failure to grasp the definiteness of 

 quantitative physics, whose natural expression is found in higher 

 mathematics. Both of these sciences are fundamental to that of 

 petrologv ; and as mathematics is the language, or expression, of 

 quantitative relationships, the more definite the knowledge of the 

 quantitative factors and relationships obtaining in igneous rocks, the 

 more natural will become their expression in mathematical terms. 



Acknowledging the usefulness of such terms as " consanguinity " 

 and " parent " magmas, in emphasizing the fact that there is relation- 

 ship between rocks in certain instances, it must be admitted that the 

 too frequent use of these and other biological terms, as " families " 

 ' Harker, A., " The Natural History of Igneous Rocks," 1909, p. 366. 



