C LA SSIFICA TION OF IGNEO US RO CKS 639 



is to be remembered that there were no specific distinctions 

 among the feldspars. It is equally true at this day that no 

 specific distinctions can be made with the unaided eye among 

 the lime-soda-feldspars, and that albite, oligoclase, andesine, and 

 labradorite cannot be identified as such without optical or chemi- 

 cal tests. It follows from this that phaneric rocks cannot be 

 classed by purely megascopical means as haying alkali-feldspars, 

 or more calcic feldspars. We must not attempt to subdivide 

 these rocks on the basis of characters not recognizable meara- 

 scopically and must content ourselves with employing mineral 

 groups, such as feldspar, mica, amphibole (hornblende), pyroxene, 

 etc., as bases for their field designation. 



For these reasons we suggest the following use of familiar 

 terms when rocks are to be named on a purely megascopical 

 basis. 



I. Phanerites — phanerocrystalline (phaneric) rocks. 



1. Granite — all granular igneous rock composed of dominant 

 quartz and feldspar, of any kind, with mica, hornblende, or other 

 minerals in subordinate amount. This is the granite of Werner, 

 von Leonhard, and other early geologists, and will include what 

 is now termed granite, granodiorite, tonalite, and most quartz- 

 diorites. It will embrace all light-colored, granular rocks with 

 dominant feldspar and a noticeable amount of quartz. It will 

 include the quartz- (hornblende) -syenites of earlier geologists. 



2. Sye?iite — all granular igneous rocks composed of dominant 

 feldspars, of any kind, with subordinate amounts of mica, horn- 

 blende, pyroxene, or other minerals, and without noticeable 

 amount of quartz. This is the syenite of Werner, von Leonhard, 

 and others, with slight modification, and will include modern 

 syenite, anorthosite, and the more feldspathic monzonites, 

 diorites, and gabbros. If it is desirable to distinguish the 

 plagioclase rocks when recognizable, such as the coarse-grained 

 anorthosites, they may be called plagioclase-syenites, or anor- 

 thosites. 



3. Diorite — all granular igneous rocks with dominant horn- 

 blende and subordinate feldspar of any kind. This is the 



