2 6 2 MATERIALS AND THEIR ARRANGEMENT 



Classifying chiefly on the basis of texture and crystallinity, there are three 

 groups: Phanerites, in which all the leading mineral constituents can be seen with- 

 out a lens; aphanites, in which at least an appreciable part of the minerals cannot 

 be distinguished by the unaided eye; and glasses, in which the material is wholly 

 or largely vitreous. 



I. The phanerites are classified further as follows: 



1. Granites, consisting largely of quartz and feldspar of any kind, with or 

 without mica, hornblende, pyroxene, or other minerals. This differs from the 

 present common use of the term granite, in not regarding mica as an essential 

 constituent, and in not distinguishing between alkali feldspars and calcic feldspars. 

 The term therefore includes more than formerly. 



2. Syenites, consisting predominantly of feldspar of any kind, with subordinate 

 amounts of hornblende, mica, or pyroxene, but with little or no quartz. This 

 differs from the common usage in giving hornblende a subordinate place, and in 

 embracing rocks with calcic feldspars. 



3. Diorites, consisting predominantly of hornblende and subordinately of 

 feldspar of any kind, with which there may be mica, pyroxene, or other minerals. 

 This is nearly the present use, except that any kind of feldspar may be the sub- 

 ordinate mineral. 



4. Gabbros, consisting predominantly of pyroxene and subordinately of feld- 

 spar of any kind, with or without other minerals. This nearly coincides with one 

 of the various present uses of the term, except that the range of the feldspar is 

 increased. 



5. Dolerites, 1 consisting predominantly of any ferromagnesian mineral not 

 distinguishable as hornblende or pyroxene, with subordinate amounts of feldspar 

 of any kind, and with or without other accessory minerals. In other words, the 

 dolerites (deceptive) embrace diorites and gabbros when they cannot be distin- 

 guished by the eye. 



6; Peridotites, consisting predominantly of olivine and ferromagnesian minerals 

 without feldspar, or with very little. 



7. Pyroxenites, consisting essentially of pyroxene. 



8. Hornblendites, consisting essentially of hornblende. 



II. The aphanites may be non-porphyritic or porphyritic. 



(a) Non-porphyritic aphanites, if light-colored, may be classed as felsites; 

 when dark-colored, as basalts. 



(b) The porphyritic aphanites or porphyries, if light-colored, are leucophyres; 

 when dark-colored, melaphyres. They may be classified further, according to the 

 kind of phenocryst (distinct crystal) imbedded in the aphanitic groundmass, as 



Quartz-porphyries, or quartzophyres; 

 Feldspar- porphyries, or f elspaphyres (not felsophyres) ; 

 Hornblende- porphyries, or hornblendophyres; and so on. 

 These may be subclassed by color, as 

 Quartz-leucophyres, light-colored quartz-porphyries; 

 Quartz-melaphyres, dark-colored quartz-porphyries; 

 Feldspar-leueophyres; 

 Feldspar-melaphyres; and so on. 



III. The glasses are classified, according to color and luster, into obsidians 



1 Added by the authors of this work. 



