388 Geology. — On the Constituent Minerals, ^. 



formity of appearance, where particular minerals predominate, 

 as in the case of mica slate and hornblende. A very slight 

 practical acquaintance with rocks will enable students in geo- 

 logy to detect their leading characters. The conglomerate rocks 

 are more easily distinguished than the rest, being compounded 

 of fragments of other rocks, united partly mechanically, and 

 partly chemically ; these fragments are of various sizes, from 

 that of the smallest particle of gravel, to masses of many pounds 

 weight. Having thus treated, in an elementary manner, of the 

 general nature of the simple bodies which are the constituents 

 of rocks, we proceed to describe that rock, called granite, which 

 lies at the bottom of the whole series, and which is of such uni- 

 versal occurrence in all countries. 



Granite, so called from its granular appearance, is a compound 

 rock, formed of one simple mineral, quartz, and two compound 

 minerals, felspar and mica. The properties of quartz are great 

 hardness, giving abimdance of sparks. It is infusible without 

 alkalies, but with them it melts and forms glass. It is often 

 slightly combined with alumine. In granite, it has a vitreous 

 translucent appearance, appears to be the result of an imperfect 

 crystallization, and may be considered as a silicious mass im- 

 bedding imperfect crystals of felspar and mica. The various 

 proportions in which these three substances enter into granite 

 rocks, constitute a great many varieties. In some of them, the 

 grains are very small; in others, the crystals of felspar are 

 large, and more regular. Granites, of this last class, have re- 

 ceived the awkward name of porphyritic granites, a term badly 

 applied to white coloured crystals. 



Felspar, is a compound mineral, consisting of various propor- 

 tions of silica, alumina, potash, and, occasionally, soda, a small 

 portion of lime, and iron. It sometimes constitutes the largest 

 part of granite rocks. When in a perfect state it is nearly as 

 hard as quartz ; but when it decomposes, which is often the case, 

 the particles constituting the granite rock, no longer cohere, but 

 fall into the condition of gravel. The usual colours of felspar, 

 are white, and many varieties of red. This mineral sometimes 

 occurs in immense masses, when it is called compact felspar, 

 trachyte, white stone, eurite. In many countries, beds of felspar 

 are worked for the porcelain manufactories. In the State of 

 Delaware, there are extensive localities of it, as well as on the 



