SALMON — ON ROCKS. 



51 



of the earth's crust ; while many again are found so seldom, that their 

 discovery in considerable quantities is of great commercial importance, 

 and some are so rare as to make even small specimens often of con- 

 siderable value. Twelve only, or less than one-fifth of the whole, occur 

 frequently, or in abundance, in the rocks at the surface of the earth. 



XI. If the elements are distributed with such great inequality, so 

 are minerals likewise. Mineralogists describe about 700 mineral 

 species, but of these comparatively few occur frequently, or still fewer 

 abundantly. On examining rock-formations, we soon find that they 

 are made up, in on enormous proportion, of some few mineral species 

 and varieties, which are met wdth so abundantly as to justify their 

 being considered as essential parts of the earth's crust, while the great 

 body of mineral species are more or less subordinate. But there is 

 another distinction to be drawn between minerals, besides their greater 

 or less abundance. Some minerals, which occur in comparatively 

 considerable quantities, are almost always found in veins, and rarely or 

 never mixed in rochs as a constituent ingredient. These we call 

 V ein-f or m in g miRQXdlQ \ and although some of them occur frequently 

 and abundantly, and are of great geological importance, yet, as they 

 do not form the constituents of rocks, to the consideration of which 

 those papers are exclusively devoted, they do not now come within 

 our scope. In contradistinction to these minerals, which we find only 

 in veins, we term those minerals w^hich we have already referred to as 

 occurring so frequently and abundantly in rocks, rocJc-forming minerals. 

 But besides these rock-forming minerals occurring in great quantity, 

 there are others which, although they are not frequently met with, 

 and are only abundant in some unfrequently occurring rocks, and are 

 sometimes even never abundant, yet form an essential though minor 

 constituent of rocks. These we must also class in the category of 

 rock-forming minerals ; and we shall consequently notice them here, 

 although they may, on the whole, be far less abundant than many 

 minerals which we exclude, because, existing almost entirely in veius, 

 they do not belong to our present subject. 



XII. I have already stated that there are only twelve elements 

 which occur in abundance in rocks — or perhaps I should rather say 

 in the minerals which make up rocks. In considering, however, the 

 chemical nature of the rock-forming minerals, it will be necessary to 



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