GEOLOGY. 



but the three just described. They are the essential, 

 and almost only ingredients in granite, gneiss, and mica 

 slate. In the oldest specimens of granite, usually of a 

 coarse texture, the three ingredients being in large 

 masses, the felspar is most abundant. The mica, fre- 

 quently in large plates, is dispersed through the mass 

 in every possible direction. As the process of forma- 

 tion continued, the felspar became less abundant, and 

 the mica more regular in its arrangement. The rock 

 hence passed from coarse to fine granite, and from fine 

 granite into gneiss. The last is slaty granite. Both 

 contain the same ingredients, and their principal differ- 

 ence is in the proportion, arrangement, and texture of 

 their ingredients. As the formation of gneiss con- 

 tinued, the felspar still continued to diminish, until it 

 wholly disappeared. When the rock is formed, it is com- 

 posed of quartz and mica finely mixed, and of a slaty 

 structure, and bears the name of mica slate. 



This rock differs from gneiss, not only in being desti- 

 tute of felspar, but in possessing a finer 'texture, and a 

 smooth, but frequently an undulating surface. 



As these three strata of rocks are more common than 

 any other upon the surface of the earth, they are used 

 for a greater variety of common purposes, ly practical 

 men. They are extensively used by farmers for en- 

 closing their fields, and by civil engineers in the con- 

 struction of roads, bridges, wharves, dams, canals, 

 railways, the walls of buildings, Sec, &c. 



Nearly allied to granite, and frequently associated 

 with it is sienite. In this rock, hornblende takes the 

 place of mica in granite ; the mass is of course com- 

 posed of quartz, felspar, and hornblende. The simple 

 mineral last named, sometimes resembles black mica in 

 its external appearance, but is much harder and can be 

 readily distinguished from it, by its resisting the point 

 of a knife, while mica is readily separated into thin 

 scales, by the application of any pointed instrument. 



Sienite is found in great abundance in the vicinity of 

 Boston, where it is at present the most common mate- 

 rial for the walls of houses and other purposes in 

 architecture. In Quincy, and two or three other towns 



