Geology. — Oti the Constituent Minerals, ^c. 389 



borders of the adjoining State of Pennsylvania, both compact 

 and decomposed. 



Mica is also a compound mineral, consisting of various pro- 

 portions of silica, alumina, potash, a considerable quantity of 

 oxide of iron, and some magnesia. It readily divides into very 

 thin and transparent laminas, or scales, with a fine silvery lustre. 

 These laminae are elastic, and in this quality differ from talc, 

 which is a mineral compound of the same nature, but whose 

 laminae, or plates, are non-elastic, although flexible. Mica is 

 always translucent, sometimes black, and in the granitic veins, 

 and beds of compact felspar on this continent, is frequently 

 found in congeries of hexagonal plates. 



The various proportions in which these constituents of 

 granitic rocks meet in mineral masses, have produced many 

 varieties of that kind of rock which lies at the bottom of the 

 geological series, and which has received the name of granite. 

 This term, however, is frequently misapplied to other rocks, 

 which do not lie at the bottom of the primary division, from 

 their having a vague resemblance to true granite. When horn- 

 blende is present, to the exclusion of mica, it generally becomes 

 a syenite, a rock which is found frequently much higher up in 

 the series, although hornblende is occasionally found in true 

 granite. When it is an aggregate, compounded of felspar and 

 hornblende, to which the French mineralogists have given the 

 name of diabase, it is a true mineralogical greenstone. (Vide p. 

 311.) Compounds, also, of felspar and quartz, are called graphic 

 granite. This mineral is found in granitic veins, and beds of 

 felspar, which, probably, are only ancient dykes of felspathic 

 matter. It has a very beautiful appearance, from the curious 

 intermixture of the felspar and quartz. The mass of the felspar 

 has a pearly aspect, with a glassy and resplendent fracture, oc- 

 casioned by a parallel polarity of all its minute portions. The 

 quartz intervenes and crosses, in a manner to present a confused 

 resemblance to ancient literary characters ; hence its name 6f 

 graphic granite. From the whole surface of the felspar, the 

 incident rays are reflected to the eye, at the same angle, with 

 very striking effect. 



When the felspar decreases in granite, and the small scales 

 of mica increase in quantity, and are arranged in layers, the 

 rock loses the massive structure belonging to granite, and be- 



