Ill THE MINEEALS CONSTITUTING EOCKS 



A rock, as previously stated, is a mineral aggregate. As a 

 rule, the number of mineral species constituting any essential 

 portion of a rock is small, seldom exceeding three or four. In 

 common crystalline limestones, the only essential constituent 

 is the mineral calcite; granite, on the other hand, is, as a rule, 

 composed of minerals of three or four independent species. 

 The mineral composition of rocks in general is greatly simpli- 

 fied by the wide range of conditions, under which their chief 

 constituents can be formed, thus allowing their presence in 

 rocks of all classes and of whatever origin. Quartz, feldspar, 

 mica, the minerals of the hornblende or pyroxene group, can be 

 formed in a mass cooling from a state of fusion; they may be 

 crystallized from solution, or be formed from volatilized prod- 

 ucts. They are therefore the commonest of minerals and rarely 

 excluded from rocks of any class, since there is no process of 

 rock formation which determines their absence. Moreover, most 

 of the common minerals, like the feldspars, micas, hornblendes, 

 pyroxenes, and the alkaline carbonates, possess the capacity of 

 adapting themselves to a very considerable range of composi- 

 tions. In the feldspars, for example, lime, soda, or potash may 

 replace each other almost indefinitely, and it is now commonly 

 assumed that true species do not exist, all being but isomorphous 

 admixtures passing into one another by all gradations, and the 

 names albite, oligoclase, anorthite, etc., are to be used only as 

 indicating convenient stopping and starting points in the series. 

 Hornblende or pyroxene, further, may be pure silicate of lime 

 and magnesia, or iron and manganese may partially replace these 

 substances. Lime carbonate may be pure, or magnesia may 

 replace the lime in any proportion. These illustrations are 

 sufficient to indicate the reason of the great simplicity of rock 

 masses as regards their chief constituents, and that whatever 

 may be the composition of a mass within nature's limits, and 

 whatever may be the conditions of its origin, the probabilities 

 are that it will be formed essentially of one or more of a half 

 a dozen minerals in some of their varieties. 



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