KINDS OF ROCKS. 443 



sparingly, through the mass, in many kinds, minnte needles 

 of hornblende, crystals or scales of biotite, magnetite ; some- 

 times nephelite, haiiynite, fcridymite. Apatite exists in the 

 rock in microscopic forms, and there is also more or less of 

 the rock in a glassy state. Sometimes contains augite, and 

 has a higher" specific gravity. Quartz-trachyte has often 

 nearly the composition of granite in which there is little 

 mica. Eruptive only. 



Varieties. — The two principal divisions under each, trachyte and 

 quartz-trachyte, are : A. Saniclin-trachyte, in which the mass is chiefly 

 sanidin ; and B. Oligoclase-trachyte, or Bomyte, in which it is partly 

 oligoclase ; but the two graduate into one another. Both occur por- 

 phyritic with tabular crystals of feldspar ; and in the latter (as at 

 the Drackenfels) the tables are sanidin. They graduate into vesicular 

 or scoriaceous trachyte. 



Trachyte, according to Reyer and Suess, occurs in the region of the 

 Euganean Hills of Tertiary, Cretaceous and Jurassic age ; and the 

 f elsyte of Paleozoic age is often hardly distinguishable from it, while 

 identical with it in composition. 



Trachyte and quartz-trachyte graduate also into felsyte-like volcanic 

 rocks of like constitution, porphyrinic or not so. The latter sometimes 

 shades into rocks of semi-glassy nature called 



14. Pearlstone, when somewhat pearly in lustre; Pitchsto^e 

 when having a pitch-like lustre; and these into the glassy 

 volcanic material called Obsidian. These glassy rocks often 

 contain spherules which are concretions consisting of feld- 

 spar with some quartz. Pumice is a light, porous, feldspa- 

 thic scoria, with the pores capillary and parallel. Ordina- 

 ry obsidian, that consists chiefly of feldspar, and is hence 

 nearly free from iron, belongs here; the rest of it belonging 

 with the augitic igneous rocks. 



15. Leucityte. — A grayish rock consisting chiefly of leucite 

 in a felsitic state, with disseminated leucite crystals. Occurs 

 at Point of Bocks, Wyoming Territory, according to King 

 and Zirkel. It differs from amphigenyte, in containing no 

 pyroxene, or only traces of it. 



3. MICA AND SODA-LIME-FELDSPAR ROCKS. 



I. NOT CONTAINING NEPHELITE. 



1. Hemi-dioryte. (Mica-dioryte, Soda-granite.) — A gran- 

 ite-like rock, in which the feldspar is chiefly oligoclase ; it 

 contains much biotite, with usually some quartz, and often 

 some hornblende. Occurs at Stony Point, on the Hudson, 



