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Show 122 GEOLOGY OF THE HIGH PLATEAUS. the lavas have emanated from primordial rocks, what are they ? There js one great group of lavas which quickly furnish ground for suspicion. Kecurring here to the generalization that the materials composing the stratified rocks have been ultimately derived from primordial matter, it is but an identical proposition to say that the chemical constitution of that primordial matter ought inferentially to be such as would yield the materials of the sedimentary rocks. It ought to possess the same constituents, and ought also to contain them in substantially the same proportions as the average constitution of the stratified rocks taken as a whole category. In a word, it should be what some biologists might call a synthetic or comprehensive type of rock, from which the stratified materials might be differentiated by the known processes of sub-aerial decomposition and selection. Secondly, it ought not to conform in composition to any one variety of stratified rock, unless, perchance, in some rare exceptional cases. Thirdly, it ought to be a very abundant and voluminous rock, erupted at almost any geological age or period, from the present as far back into the past as we are able to discriminate the age of an eruption. Among the several groups or sub-groups of volcanic rocks do we find any one of them answering to this ideal type % This question does not admit of a very brief and decisive answer. We have no very accurate knowledge of the mean constitution of the stratified rocks. There is a statement, handed down, I believe, from Bischof, and passing current in the text-books, that silica constitutes very nearly 50 per cent, of the mass of all known rocks, and the estimate seems to be a very fair one. Its probable error is certainly small if the impressions of the geologists who have given much attention to lithology are to be trusted. This percentage of silica is substantially the same as that found in the basalts, and if there be a synthetic type of eruptive rocks this fact fastens suspicion at once upon the basaltic group. Probably no lithologist will hesitate to say that next to silica the most abundant constituent of the stratified rocks is alumina; but the exact proportions we do not know. Alumina is, however, known to be the second in quantity in the constitution of average basalt. But the third constituent of basalt in respect to quantity is iron oxide; in the foliated rocks it is unquestionably lime. Here is a discrepancy, and a well-marked one, which we cannot |