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Show 124 GEOLOGY OF THE HIGH PLATEAUS. » But not only should we infer that the primordial masses of the earth (or "primitive crust") were basic like the basalts or dolerites, but that they were very nearly homogeneous. If we are at liberty to speculate at all upon the physical condition of an all-liquid planet, its molten surface exposed to radiation and to the action of its immense atmosphere, we should be led to infer that it would be agitated by disturbances similar in nature, though inferior in magnitude, to those affecting the sun, thus producing a thorough and homogeneous mixture of the compounds of silica with alumina, the earths, and alkalies. This admixture once formed would, so far as we can now see, remain unaltered until it cooled sufficiently for the reactions of the atmosphere. We know of no natural processes capable of separating the more acid parts of such a magma except the chemistry of the atmosphere acting at temperatures far below the melting-points of the silicates. We have the results of that process in the quartzites, granites, gneisses, and syenites among the siliceous rocks; and the limestones and dolomites among the basic rocks; with argillaceous rocks as the residuum of the decomposition. Yet if these rocks could be remelted together they would form one homogeneous magma. Every iron-smelting furnace is an experimental demonstration of the tendency of silica to take up and hold at fusion-temperature alumina, lime, magnesia, potash, and soda in proportions exceeding those which occur in nature. No facts are known to me which justify the conclusion that segregation into two magmas could occur in such a state of fusion. Nor would it be of any service in this connection to establish the possibility of such a segregation.* It is suggested by Mr. King that crystals might form in the liquid and sink by reason of their superior specific gravity. Although I hold it to be extremely doubtful whether any crystals are formed while the rocks are melted, and very probable that the greater part of them are formed during the viscous stage of cooling (especially the hornblendes and pyroxenes), there is one consideration which would prevent us from using this view to predicate a theory of a single magma separating into two or more of very different degrees of acidity. The low percentage of silica in basalt is due not * Iron, however, might separate from such a compound, either as a regulus or as magnetic oxide, if the conditions were favorable and the oxide in excess. |