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are mostly composed of felspar, quartz, and mica. Of 

 those three substances, the felspar and mica, are, from 

 the character of their component parts, most liable or 

 subject to decomposition : the former in particular, 

 when it contains a portion of potash, as it sometimes 

 does ; and the latter when exposed by itself to all the 

 vicissitudes of temperature or weather But when the 

 felspar is of the common kind, and free from potash 

 or any other substance out of the common order, calcula- 

 ted to promote a decomposition, and when the three sub- 

 stances are perfectly combined, so as to form compact 

 granite, or any of its subordinate divisions, as gneiss, 

 sienite, mica slate, &c. but very little appearance of 

 decomposition takes place, though exposed to the chan- 

 ges of seasons for ages in succession, as I shall en- 

 deavour to prove in the sequel. 



The only substance, then, that comes under the de- 

 nomination of granite, most subject to decomposition, 

 is the coarse imperfectly formed micaceous schistus, or 

 schistose mica, in which the mica predominates, and is 

 very commonly intermixed with sulphuret of iron, 

 which, by its decomposition, promotes that of the 

 whole mass. This substance commonly occurs 

 among the strata of gneiss, and, except in some 

 instances, is not very abundant. How little then 

 can be derived from this source, towards the filling 

 np of valleys, or the formation of alluvial dis- 

 tricts ? More particularly so, since neither this, nor 

 either of the other three classes of rocks, are subject 

 to decomposition, except being actually exposed to the 



