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Show CONSIDEKATION OF APPAKENT EXCEPTIONS. 137 to cease.*fc If, then, these facts will bear the interpretation which I have placed upon them, we have in the rocks themselves the evidence required to show that propylite is a rock which at a certain temperature is just sufficiently fused and just sufficiently expanded to fulfill the mechanical conditions requisite for eruption. It still remains to look at some points in the application of this theory to the succession of eruptions, which would at first sight appear anomalous if not inconsistent with it. We do not always find the order of succession heretofore described to have been strictly followed; we find exceptional cases. Instances are not wanting where true basalts have outflowed prior to the eruption of rhyo-lites, and are even known to be overlaid by trachytes in the Auvergne district of France, or as Lyell has found to be the case in the Madeira Islands. These, however, seem to be exceptional instances. Even in the Auvergne and Madeiras the great preponderance of occurrences conform to the observed law of Richthofen, and so far as our knowledge of other regions extends the departures from this law are not common. But it may be asked whether a single unequivocal exception is not sufficient to seri- -ously impair, if not wholly break down, the explanation of the sequence here given. So far are they from impairing it, that I think a, little examination will show that not only ought we to look for exceptions, but we may even be surprised that exceptions have not been found more numerous than they appear to be. In the brief explanation given it has been assumed tacitly, that the rise of temperature has been uniform or followed some definite law of variation throughout the entire field of subterranean magmas. In its simplest or typical form the proposition assumes that in all typical or normal cases the rise of temperature affects all parts of this field alike. But this we could not expect. It is not probable that a uniform rise of temperature would take place in all parts of the field, but may vary * It was when I was contemplating the great distances traversed by slender basalt streams in Southern Utah that this theory suggested itself to me. I could not doubt that such lavas must have been ejected at a temperature much more than sufficient to melt them. This seemed to contrast powerfully with the habits of trachytic masses. It occurred to me then that this high temperature might be absolutely essential to the eruption of so dense a rock as basalt, while a considerably lower one would suffice for lighter rocks. Immediately the higher melting temperature of the rhyolites and trachytes suggested itself, and almost as quickly as I write it the theory took form in my mind and the double function of density and fusibility associated itself with the double sequence. |