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Show COMPARISON OF ERUPTIVE WITH METAMORPHIO ROCKS. 117 accepted theory. But the relations between the metamorphic and eruptive rocks constitute a pending question. It will be unnecessary here to enter very minutely into a discussion of these relations, and, indeed, a full discussion would require a very long and copious review of the existing state of lithological science. It will be sufficient to state in a summary manner those points of comparison which immediately concern the subject in hand. The conclusion to which this comparison tends is that a large proportion of the igneous rocks have the petrographic characters which we ought to expect would result from the fusion of certain groups of metamorphic stratified rocks. There are three points of view from which the comparison may be made; these are with reference, first, to chemical constitution; second, to mineral components; third, to mechanical texture. 1st. Metamorphic and igneous rocks compared with respect to chemical constitution.â€"The eruptive rocks are highly complex compounds, and always contain certain constituents which may be called essential constituents. These are silica, alumina, lime, soda, potash, and magnesiaâ€"six in number. Iron in the form of some oxide is almost always present, but since it is occasionally absent, or found in exceedingly small quantity, it cannot be regarded as a universal and essential constituent. Silica is always the dominant ingredient, and though the quantity of it varies greatly, yet the variation is within tolerably definite limits, almost never exceeding 80 per cent., and almost never falling below 45 per cent. The remaining five constituents likewise vary, but always within tolerably narrow limits. Thus alumina rarely falls below 13 per cent, and rarely exceeds 26 per cent. Lime rarely exceeds 14 per cent, magnesia 10 per cent, soda 9 per cent., and potash 8 per cent. The variations in the relative proportions of these constituents is sufficiently wide to give well-marked specific or even generic differences in the kinds of volcanic products; but the variations are so limited and the relative proportions subject to such moderate departures from normal ratios, that the whole category of eruptive rocks possess at least ordinal if not family likenesses. Turning now to the metamorphics we find a far wider range of chemical constitution. Thus we have quartzites which are almost pure silica; we have crystalline limestones and dolomites |