70 New York State Museum 



luster. With a reduction in quartz the rock approaches 

 syenite, which consists typically of orthoclase feldspar 

 and hornblende, and when there is an increase in acid 

 plagioclase feldspar, quartz diorites or diorites are ap- 

 proached. Quarts diorite resembles granite but is darker 

 and heavier. Diorites differ from the above in having 

 no quartz. A granite in which feldspar crystals are of 

 large size and distinct, standing out against the finer 

 granular background, is called porphyritic granite. Peg- 

 matite dikes are so common in granite that, unless other- 

 wise specified, whenever the term is used granite is meant. 

 Pegmatite is a very coarse variety of granite occuring in 

 veins of dikelike masses. The chief minerals are quartz, 

 feldspar (usually orthoclase) and muscovite mica. The 

 crystals are sometimes a foot or more in diameter and 

 it is from this source that mica for commercial purposes 

 is obtained. The quartz and feldspar are sometimes so 

 intergrown that upon the feldspar surfaces the quartz ap- 

 pears as dark irregular masses resembling Arabic writ- 

 ing. This is known as graphic granite. 



The basic igneous rocks are all included in the gabbro- 

 basalt group. Gabbro is a coarsely granular dark rock 

 chiefly composed of basic plagioclase feldspar and pyrox- 

 enes, though hornblende, olivine and biotite may be pres- 

 ent in certain varieties. With a decrease in the feldspar 

 and an increase in the ferromagnesian minerals the gab- 

 bros pass over into other rocks, pyroxenites etc. Basalt 

 is the name given to the fine-grained or dense basic igne- 

 ous rocks derived from surface flows. Diabase is a fine- 

 grained, wholly crystalline rock which stands between the 

 gabbros and basalts. It is common as an intrusive rock 

 in dikes or sills. In the field the name dolerite is applied 

 to dark rocks which may be either gabbros or diorites 



