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Show THE EXPLANATION OF THE SEQUENCE. 133 density upon silica, but it is still known that there exists an approximation to such a dependence. This will also be subsequently alluded to. A curve may be constructed, as before, representing this dependence, which may be called the curve of fusion. Since both density and fusion have approximate relations to the quantity of silica present (and for present purposes such relations are assumed to be exact), they are functions of each other. We know that with increasing percentages of silica the density diminishes, while the melting temperature increases, and hence the two curves if indefinitely prolonged will somewhere intersect. It remains to determine, if possible, the point of intersection. Let us for the present arbitrarily assume that the point of intersection is such that both curves have a common ordinate erected from a point on the axis of abscissas corresponding to GO per cent, of silica, which is very nearly the normal percentage of horn-blendic propylite. I shall hereafter adduce reasons for believing that this arbitrary assumption is very nearly or quite true. We have now (ex hypothese) two curves, one representing the temperature required to render the rocks light enough to rise hydrostatically to the surface, the other representing the temperature required to fuse them. Conceiving, then, a general rise of temperature to occur among subterranean groups of rocks, no eruption could take place at any temperature less than that represented by the ordinate drawn at 60. For the basic rocks would still be too dense, while the acid rocks would be unmelted. But when that temperature is reached, the propylite would be in an eruptible condition. By a further increase of temperature hornblendie andesite and trachyte would become eruptible, the former having passed the fusion point and the latter having passed the density point of eruption. And in general as the temperature increases the line of eruptive temperature cuts the two curves at points further and further from the lowest point of eruptivity, and these points correspond to rocks which become more and more divergent in their degrees of acidity; one set progressing to the acid extreme, the other to the basic extreme. If now our fundamental assumptions are true, or in essential respects conform approximately to the truth, then the sequence of eruptions which those assumed conditions would give rise to conforms to the sequence which we find in nature. Let us, then, examine these |