25d COMPOSITION OF GRANITE, FELSPAR, AND ALBITE. 



Scilly Isles, as to be considered characteristic of a very large portion of 

 them (Dr. Boase). 



These rocks decay with very different degrees of rapidity — accord- 

 ing to the proportions In' ti^hich the severa. minerals are present in 

 them, and to the peculiar state of hardness or aggregation in which they 

 happen to occur. Both the mode of their decay, however, and the cir- 

 cumstances under which it takes place, as well as the character and 

 composition of the soils formed from them, are materially dependent 

 upon the composition of the several minerals of which the rocks consist. 

 This composition, therefore, it will be necessary to exhibit. 



1^. Quartz has already been described (p. 206), as a variety of silica 

 — the substance of flints, and of siliceous sands and sand-stones. In 

 granite, it often occurs in the form of rock crystal, but it is more frequent- 

 ly disseminated in small particles throughout the rocky mass. It is 

 hard enough to scratch glass. 



2*^. Felspar is generally colourless, but is not unfrequently reddish or 

 flesh-coloured. On the colour of the felspar they contain, that of the 

 granires most frequently depends. Several varieties of this mineral are 

 k?io\vn to collectors. Besides the common felspar, however, it is only 

 necessary to specify Albite, which, in appearance, closely resembles fel- 

 spar, often takes its place in granite rocks, and in chemical constitution 

 differs from it only in containing soda, while the common felspar con- 

 tains potash. These two minerals are readily distinguished from quartz 

 by their inferior hardness. They do not scratch glass, and, in general, 

 may easily be scratched by the point of a knife. 



They concitt respectively of — 



Felspar. Albite. 



Silica .... 65-21 69-09 



Alumina . . . 18-13 19-22 



Potash .... 16-66 — 



Soda .... — 11-69 



100-00 100-00 



It is to be observed, however, that these minerals do not generally oc- 

 cur in nature in a perfectly pure state — for though they do not essential- 

 ly contain either lime, magnesia, or oxide of iron, they are seldom found 

 without a small admixture of one or more of these substenees. It is also 

 found that while pure felspar contains only potash, and pure albite only 

 soda, abundance of a kind of intermediate mineral occurs which contains 

 both potash and aoila. Such is the case with the felspar of the Siebenge- 

 hirge, on the right twrnk of the Rhine (Berthier), and with those con- 

 tained in the lava^ of Vesuvius and the adjacent parts of Italy (Abich) 



In these two mmerals the silica is combined with the potash, soda, 

 and alumina, forming certain compounds already described under tlie 

 name o^ silicates (p. 207). 



Felspar consists of a silicate of alumina combined with a silicate of 

 potash. Albite of the same silicate of alumina combined with a silicate 

 of soda. 



3^. Mica generally occurs disseminated through the granite in smnil 

 shining scales or plates, which, when extracted from the rock, split rendi- 

 ly into numerous inconceivably thin layers. I: sometimes occurs also 



