204 UNSTRATIFIED OR IGNEOUS ROCKS. 



a crystalline aggregate of clinic or soda-lime feldspar plagioclase), and 

 hornblende; and, therefore, differs from syenite of German writers 

 only in the form of the feldspar — viz., plagioclase instead of ortlioclase. 

 When quartz is present it is called quartz-diorite. 



Diabase, — This is a dark, greenish crystalline rock, usually fine- 

 grained, but sometimes granitoid, somewhat similar in appearance to 

 diorite, but differing in the fact that augite replaces hornblende. It 

 also often contains olivin. Gabbro is a granitoid variety of diabase, in 

 which the augite takes the form of diallage. 



We have selected these as good types of the groups ; but they merge 

 insensibly into each other, giving rise to many varieties, for the de- 

 scription of which we must refer the reader to special treatises on 

 lithology. 



Diorite and diabase are so frequently intrusive and fine-grained 

 that they are often treated in an intermediate or even in the second 

 group ; but they also often occur massive. 



Two Sub-Groups — Acidic and Basic. — Quartz is pure silicic acid. 

 Feldspar is a silicate of alumina and alkali, with excess of silica — i. e., 

 an acid silicate of these bases. In orthoclase the alkali is potash ; in 

 plagioclase, soda and lime. Moreover, the former is more acid than 

 the latter. Hornblende and augite are basic silicates of somewhat 

 similar composition. Augite is essentially a silicate of magnesia and 

 iron ; while, in hornblende, alumina and lime replace a portion of the 

 magnesia. Kemembering, further, that quartz and feldspar are light- 

 colored minerals, with specific gravity of about 2*6, while augite and 

 hornblende are usually black minerals, with specific gravity of 3 to 3*5, 

 it is plain that this group of rocks may be divided into two sub-groups, 

 acidic and basic, often recognizable to the eye. In the one there is a 

 predominance of quartz and feldspar, in the other of hornblende or 

 augite. Also, in the one the feldspar is orthoclase, in the other plagi- 

 oclase. The one is light colored, of less specific gravity, and more 

 difficultly fusible ; the other darker colored, heavier, and more easily 

 fusible. Granite is the best type of the one; diorite, and especially 

 diabase or gabbro, of the other. Syenite is intermediate. The per- 

 centage of silica, both free and combined, in granite is 62 to 81, and 

 the specific gravity 2'6 to 2-7. The silica percentage in diabase is 45 

 to 56, and the specific gravity 2*7 to 2*9 (Von Cotta). 



Mode of Occurrence. — True Plutonics, especially of the granitic 

 type, such as granite and syenite, occur : 1. In large masses, forming 

 the axes of great mountain-ranges, such as the Sierra and Colorado 

 ranges (Fig. 182, A) ; or, 2. In rounded masses, appearing in the midst 

 of stratified rocks like islands in the midst of the sea (Fig. 182, B) ; or, 

 3. Sometimes in tortuous, irregularly branching veins, extending only 

 a little way from the great masses into the overlying stratified rocks, as 



