8 4 SCIENCE PROGRESS 



Conclusions 



The present trend of penological thought is toward an 

 increasing recognition of the quantitative element in the science. 

 A quantitative classification is demanded merely on the grounds 

 of utility, especially for the purpose of comparing and correlating 

 igneous rocks, for which qualitative descriptions afford only a 

 very vague and inconclusive basis. With the exception of 

 chemical and mineralogical composition, all the bases of classifi- 

 cation hitherto proposed are insusceptible to the quantitative 

 method. A quantitative classification based on chemical com- 

 position is a desideratum for various purposes, but is unsuitable 

 for the everyday working needs of petrographers, because the 

 chemical analysis of a rock usually takes at least a week of the 

 petrographers time, and it is therefore impossible for him to 

 obtain data for all the rocks he wishes to classify. The mode 

 of a rock is much more easily obtained, and is quite as sus- 

 ceptible to the quantitative method as the chemical composition. 

 The main lines of a suitable modal classification are already 

 formulated in the second volume of Iddings' great work on 

 Igneous Rocks. With a little modification in detail, and 

 elaboration on the scale and with the method of the American 

 Quantitative Classification, it is believed that it will satisfy the 

 immediate needs of petrographers. If and when they are 

 elucidated, the natural or genetic relations of igneous rocks 

 will be as easily expressed in terms of the quantitative modal 

 classification, as various physical relations are expressed in 

 terms of the artificial and arithmetically bounded units of the 

 scale of temperature and other physical properties. 



