SALMON — ON ROCKS. 
51 
of the eartli's crust ; while mauy again are found so seldom, that their 
discovery in considerable qiiantities 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 itiinerals 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 an enormous proportion, of some few mineral species 
and varieties, which are met with so abundantly as to justify their 
being considered as essential pai-ts 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 rocTcs as a constituent ingredient. These we call 
vein-forming minerals ; 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 
these 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 which we have already referred to as 
occurring so frequently and abundantly in rocks, roc^-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 unfi-equently 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 veins, 
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 necessaiy to 
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