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Show 134 " GEOLOGY OF THE HIGH PLATEAUS. assumptions, with a view to ascertaining, as well as we are able, how nearly they approach the truth. 1st. It is assumed that the density is some approximately definite function of the percentage of silica. There are indeed considerable variations from exactness in this respect, and we may select two or more species of rock having the same silica contents, but which differ conspicuously in density. Yet nothing is more certain than the fact that as a general rule the assumption is very near the truth. This is so well known that further discussion is probably unnecessary. 2d. It is assumed that melting temperatures also bear an approximately definite ratio to the silica. Here the variations from exactness are no doubt somewhat greater than in the case of density. Still, we know that on the whole the law strongly prevails, and that the melting temperature diminishes with the acidity of the rock.* The blast-furnace slags present often very close approximations to many of the volcanic rocks, and these approximations are not infrequently so close as to be fairly comparable. In such cases it is familiar to those who are acquainted with the practical working of furnaces that the more basic slags are much more easily fused than the more acid ones. The absolute melting temperatures, however, are not accurately known. 3d. The assumption that the two curves (density and fusion) will ordinarily cut each other at the ordinate of 60 per cent, of silica is one which presents greater difficulty. Translating graphical terms into concrete language, the meaning of it is this: It assumes that rocks having a normal percentage of about 60 per cent, of silica, and corresponding lithologically to the hornblendic propylites are fused and rendered light enough to erupt at one and the same temperature; while rocks more basic are fused at a lower temperature, but require a higher one to be sufficiently expanded ; and rocks more acid are sufficiently expanded at a lower temperature, but require a higher one to fuse them. Is there any independent evidence of the verity of this assumption 1 The point is a very important one; indeed, vital. For if the intersection of the two curves be elsewhere, *Sec observations of Biscliof ou fusion of igneous rock, D'Archiac, vol. iii, and results of Devillo and Delesse, Bui. Soc. Gcol. France, 2d ser. iv. D. Forbes Cbcm. News, xviii. |