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Show NOT ALL LAVAS AEE PEODUOT OF BEFUSION. 121 approximate identity of chemical constitution is what we should anticipate on that assumption. We should expect to find some minerals common to both classes of rocks, while other minerals are found in one class alone. We should look for nothing but contrast in the respective mechanical textures ; and we find the anticipated agreements and contrasts. But there is an important consideration which will not permit us to conclude that all eruptive rocks are derived from the fusion of metamor-phics; for whence came the materials of the metamorphic rocks themselves I Accepted theories declare that their ultimate origin was in the primordial- materials of the earth-mass, which were broken up, decomposed, and the several components sorted out and arranged in the form of sediments; and these sedimentary formations gradually accumulated until they completely buried the primordial mass, so that no portion of it is anywhere exposed, so far as has yet been discovered. But when the primitive mass was finally buried, from what sources could the materials have been derived which could add fresh layers to the covering 1 To this there is but one possible answer. After the greater portion of the original surface had been covered, additional sediments must have been derived from the extravasation of primordial matter. This conclusion seems to be logically perfect. In the past epochs these primitive materials must have been continually extravasated, though, as the body of sedimentary formations increased, it is possible that they too began to be erupted by secondary fusion, and with the lapse of time formed an increasing proportion of the total extravasation, while the proportion of primitive matter as gradually diminished. Now, have we any reason for supposing that the evolution of the earth has so far advanced that primitive matter has ceased to erupt, and that modern outbreaks consist wholly of materials which had once before in the world's history been poured out, broken up, decomposed, stratified, metamorphosed, and again erupted 1 If so, then the body of stratified rocks is no longer increasing, but the revolutions of time are simply working over the stratified rocks again and again. But this is improbable in a high degree. There is no warrant whatever for such a belief, and therefore no justification for the inference that all eruptive rocks are derived from the secondary fusion of the metamorphics. But if it is probable that some of |