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Show 116 GEOLOGY OF THE HIGH PLATEAUS. sistent with the assumption that lavas are portions of a primordial, uncon-gealed earth-liquid, forming either a general fluid nucleus or extensive isolated vesicles. They point rather to many small reservoirs, situated at no very great depths, each of which contains, not a primordial liquid, but a liquid secreted, so to speak, from surrounding rocks, or generated by a secondary and progressive fusion of solidified matter occurring in macula within the layers of the rocky envelope of the earth. The whole tenor of volcanic phenomena bespeaks a process which is extremely localâ€"a process which has an inception, a growth, a culmination, a decadence, and a final cessation, all within a limited and rather small area and determined by some local cause. But we find the strongest evidence against the hypothesis that lavas are primordial liquids when we come to the study of their physical, chemical, and mineralogical characters. We do not, indeed, have any very decisive grounds for asserting what the primordial liquids might consist of or what would be their petrographic characters if any of them were erupted to the surface, and so far we might not be justified in saying that the lavas from volcanoes are distinct from them. But there are some eruptive masses which are very plainly not primordial. For instance, a decidedly conspicuous mass of these products are not fused rocks, but hot mud holding large quantities of rocky fragments, which have unmistakably formed the clastic components of strata. The volcanoes of Central America and the Andes and of the Batavian Islands have within the last century disgorged astounding masses of hot mudâ€"material which has not been fused at all, but rendered plastic and capable of flo\? by the combined action of heat and watery solution. It cannot be admitted that such erupta can have come from primordial materials. And the indications are no less distinct that the greater part of the true lavas have originated from other sources. The careful and systematic study of the petrographic characters of all rocks, whether sedimentary, metamorphic, or eruptive, has enabled us to compare them intelligently, and to form some conclusions as to the homologies on the one hand and the distinctions on the other which exist between them. The great generalization that the foliated crystalline rocks are altered sediments has long since passed into geological science as a fully |
