| OCR Text |
Show 90 GEOLOGY OF THE HIGH PLATEAUS. quantity of this constituent is second only to that of silica, it varies less than any other. It rarely falls below 14 per cent, and rarely exceeds 19 per cent, of the entire rock. There is a tendency to a slight excess of alumina above the quantity required to form feldspar in the acid rocks and a tendency to a slight deficiency for the formation of feldspar in the basic rocks.* Hence the slight excess of alumina of the acid rocks may readily be taken up by the aluminous micas and aluminous hornblende; and in the basic rocks, on account of the deficiency of alumina, the lime cannot all take the form of feldspar, and a considerable portion of it appears in the very abundant augite. Thus we find that basic rocks have basic minerals and acid rocks have acid minerals, and that the mineral ingredients stand in correlation to the chemical composition of the magma, and that the nature of the latter is a determinant of the former. Perhaps the most striking example is to be found in the varying conditions which determine the formation of augite and hornblende. These two minerals differ but little in chemical constitution, and yet their slight differences are distinctly correlated to differences in the composition of the magmas from which they crystallize. In augite, lime and iron are found in greater quantity and alumina in less quantity than in hornblende. Although the differences in these respects are rather small, they appear to be strictly proportional to correlative differences in the general groundmass in which they respectively occur. Correlation between chemical composition and specific gravity.â€"The existence of such a correlation is perhaps too well known and too obvious to require any discussion. In general the density holds an inverse ratio to the acidity. Correlation between the chemical composition and fusibility.â€"The fusibility of volcanic rocks has not been investigated so fully as other properties, and neither lithologists nor geologists appear to have attached any very great *The percentage of alumina, however, is less in the acid than in the basic rocks, and yet the excess above the quantity required to form soda and potash feldspars is usually greater in the former rocks than in the latter, on account of the great acidity of the alkali feldspars; indeed, there is rarely any notable excess of alumina in the basic rocks above what is required for the basic lime-feldspar. Thus the rocks which have the smaller percentage of alumina curiously enough have an excess above the requirements of feldspar, and it appears in the accessory minerals, while the rocks which have the higher percentage are rather deficient in it. |
