CLA SSIFICA TION OF IGNEO US RO CKS 637 



these, applicable only to the generic name, but will be applicable 

 to any division of the classification from Class to Subgrad. 



ROCK NAMES FOR GENERAL FIELD USE. 



It is obvious that a considerable part of the system of clas- 

 sification and nomenclature here proposed can only be applied 

 upon microscopical or chemical investigation. This is equally 

 true of a large part of that in present use. There are many 

 distinctions based on characters that cannot be observed with 

 the unaided eye. such as differences among the plagioclase feld- 

 spars, and the mineral composition of aphanitic rocks. 



It is also clear that the system demands a more detailed 

 knowledge of rocks than many geologists, mining engineers, and 

 others interested in geology, may care to acquire. This is a 

 natural consequence of the advance in petrological science, 

 which requires a corresponding advance in specialization and in 

 petrographical classification and nomenclature. 



For these reasons, which might be elaborated at consider- 

 able length, we are convinced of the need of general petro- 

 graphical terms which will be serviceable in the field work of 

 the petrologist, and which will be of use to the general geologist 

 and to those who may not be able to carry on microscopical and 

 chemical investigation. 



Such general terms should be based on megascopic char- 

 acters of the rocks and should be limited to such characters. 

 Their application should be purely objective and everything 

 of a subjective nature should be eliminated. It follows from 

 this that such terms cannot be correlated with those used 

 in the systematic nomenclature based on chemical and micro- 

 scopical properties. They must be understood to have a totally 

 different scope, and to indicate no more than the general mega- 

 scopic characters with which they are connoted. 



The attitude of the person using such terms is the same as 

 that of geologists who studied rocks before the introduction of 

 the microscope, and most of the distinctive features exhibited 

 megascopically by igneous rocks were well known and appro- 



