MINERALOGICAL CLASSIFICATION OF IGNEOUS ROCKS 87 



Rockallite 215(2). 



Rougemontite 2415(1). 



Rouvillite 2225(1). 



Shonkinite 2112(1), 3112(1), 3212(1). 



Solvsbergite 2112(2), 2113(4), 2123(2). 



Syenite 2111(2), 2113(3), 227(11), 228(7), 229(1), 2212(2), 2312(1), 



2313(1), 3212(1), 327(1). 

 Tawite 2127(1). 



Tinguaite 2122(1), 2123(4), 2124(3), 2116(1). 

 Trachyte 216(1), 2113(1), 2123(1), 2212(1), 2213(2). 

 Vulsinite 2213(1), 2222(1), 2223(2). 

 Yamaskite 3415(3). 



CLASS NAMES 



In a few cases the old classifications give special names to the 

 dark varieties of feldspathic rocks. Thus shonkinite was definitely 

 defined as a syenite with more than half of the constituents dark, 

 although in the foregoing list one rock (2 11 2) is mesocratic. In 

 most cases, however, there are no special names for the dark 

 feldspathic rocks, nor is it necessary to invent such, for the differ- 

 ent varieties may be distinguished by prefixes. Since the rocks of 

 Class 4 are separated from each other on an entirely different basis 

 from the rocks of the other three classes and have special names 

 they need not be considered here. To the other three classes the 

 names suggested by Brogger — leucocratic, mesocratic, and melano- 

 cratic — may be prefixed. If desired, a rock may be called a leuco- 

 granite, meso-granite, or melano-granite, for example, instead of a 

 leucocratic granite, mesocratic granite, etc. Meso, unfortunately, 

 has been used as a prefix for Mesozoic rocks, but since the age 

 classification of igneous rocks is no longer in use this would cause 

 no confusion. Furthermore, since the normal rock usually falls 

 in Class 2, the meso prefix is seldom necessary, and its name may 

 be used without a prefix. 



ORDER NAMES 



The different orders may be indicated, when no special names 

 exist for the various rocks, by the prefixes albite- (or soda-), sodic-, 

 calcic-, and anorthite- (or lime-). Thus in the diorite family the 

 rocks of the different orders would be albite- (or soda-) diorite, 



