CLA SSIFICA TION OF IGNEO US RO CKS 635 



Type and Habit. — It is clearly obvious that if great pre- 

 cision or completeness be desired, so that both mineralogical 

 and textural qualifiers are used, the polynomial name resulting 

 will be of considerable length, comparable to such present 

 names as quartz-hornblende-biotite-diorite-porphyry. This will 

 probably not be as great a practical difficulty or inconvenience 

 as may appear at first sight, since after a given rock has been 

 described and named in full in any given article, it may be 

 referred to subsequently by its magmatic name alone, or by this 

 in conjunction with a textural or modal term, according to 

 circumstances. 



However, since the same or similar assemblages of modal and 

 textural characters are found in many localities it will be as well 

 to be able to express these concisely. This is, after all, what is 

 accomplished by many of the names in the present systems of 

 nomenclature, although it has not been done systematically. 

 Thus the names granite, rhyolite, tinguaite, laurvikite, etc., 

 convey primarily an idea of the qualitative mineral composition 

 and the texture of the respective rocks, with a very rough one 

 of the magmatic character. 



It is to be noted that there are two degrees of similarity 

 among rocks which can be easily recognized and made use of. 

 One is almost complete identity, the other a ge?ieral resemblance 

 which suggests identity. 



Type. — For the first of these, identity or almost complete 

 identity, we propose the use of the term type. Rocks of the 

 same type are identical in norm, mode, and texture, or almost 

 completely so. They are of the same grain, have the same 

 fabric, the same actual mineral composition, and are so much 

 alike that they may be mistaken for one another or might have 

 been parts of one rock body. Many examples of such close 

 similarity are familiar to all petrographers. 



The particular modal and textural features which characterize 

 rocks of a given type are to be expressed by a single adjective 

 word, composed of a root derived from a geographical locality 

 in which a rock of the type occurs, but not already employed 

 to designate a magmatic unit, and the termination -al. 



