212 STRUCTURAL GfiOLO&Y. 



texture, and have a speckled or mottled appearance, be- 

 cause composed of crystals of considerable size, and of 

 different colors, aggregated together. The crystals of 

 which they mainly consist are, quartz, feldspar, mica, 

 and hornblende. In such a coarse, speckled rock, the 

 bluish, glassy, transparent spots are quartz ; the opaque, 

 whitish, or rose or greenish crystals, with striated surface, 

 are feldspar ; the black spots are usually hornblende ; the 

 mica may be known by its thin, scaly structure, some- 

 times pearly, sometimes black. 



The whole group is called granitic, because granite is 

 its best type. In popular language, indeed, all these rocks 



would be called granite, but sci- 

 ence makes a difference. If the 

 rock consists of quartz, feld- 

 spar, and mica, or else of these 

 with hornblende, then 'it is 

 granite proper. If it consists 

 of feldspar and hornblende, or 



FIG. .aphic gite. these with quartz, it is called 

 syenite. If it consists of only 

 quartz and feldspar, and the quartz be in bent plates, 

 looking, on section, like Hebrew characters, it is called 

 pegmatite (Fig. 122). The feldspar in all these is potash- 

 feldspar, or orthoclase. Diorite is a dark, speckled rock 

 of the same composition as syenite, except that the feld- 

 spar is a soda-lime feldspar or plagioclase. Gabbro and 

 diabase are dark-greenish rocks similar to diorite, except 

 that the hornblende is replaced by augite and olivine.* 



Mode of Occurrence. The mode of occurrence of 

 these rocks has been already explained. They never 

 occur in overflows. They rarely or never occur in in- 

 truded sheets or dikes. They occur only in great masses, 

 or sometimes in tortuous veins closely connected with the 



* The teacher must have a small collection of rocks and of min- 

 erals for illustration. 



