276 PRINCIPLES OF STRATIGRAPHY 



When the ground mass predominates, the pheno- 

 crysts being few, the rock is spoken of as porphy- 

 ritic rhyolite, trachyte, etc. But if the phenocrysts 

 abound then the rock is cahed a rhyoHte porphyry, 

 trachyte porphyry, andesite porphyry, etc. 



Porphyries (Cross. etc.-4) may, in a general way, 

 be divided into Hght-colored, or leucophyres, and 

 dark-colored, or melaphyres, and, according to 

 the character of the phenocrysts, into Ouarts-leu- 

 cophyrcs, Feldspar-lcucophyres, Quarts-melaphy- 

 res, F elds par-melaphy res, etc. When intermediate 

 as to color the names quartz-porphyry or qtiart- 

 zophyres, feldspar-porphyry or feldsparphyres, 

 hornblende porphyries, or hornhlendophyres, etc., 

 are useful. 

 Cryptocrystalline texture may be original or second- 

 ary through a process of devitrification of lavas orig- 

 inally glassy or acrystalline. They may be divided 

 as follows : 



B-i. With Trachytic texture, the ground mass 

 consisting of an aggregate of little rods of 

 feldspar in parallel arrangement. This is 

 sometimes visible to the unaided eye. 

 B-2. With Pelsitic texture (lithoidal), when the 

 ground mass is so dense as to appear 

 acrystalline except under the microscope, 

 where it is seen to consist of minute quartz 

 and feldspar grains with some glass. 

 B-3. Vesicular and Ainygdaloidal, with a crypto- 

 crystalline ground mass, but with numer- 

 ous vesicles or steam holes which may be 

 filled with mineral matter (amygdaloidal 

 texture). 

 II. Amorphous Division. 



C. Acrystalline, or glassy, the greater part of the mass be- 

 ing an amorphous glass, without crystalline charac- 

 ter even under the microscope. It may be divided 

 into: 



C-i. Vitreous, or obsidianic, when a uniform 



glass as in obsidian. 

 C-2. Perlitic with spheroidal structure due to con- 

 centric cracks from contraction in cooling. 

 Ex. Perlite. 



