THE EXPLANATION OP 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 

 rnay be constructed, as before, representing this dependence, which may be 

 called the curve of fusion. Since both density and fusion have approxi- 

 mate 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 in- 

 definitely 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 com- 

 mon ordinate erected from a point on the axis of abscissas corresponding to 

 60 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 tempera- 

 ture 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 subterra- 

 nean 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 hornblendic 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 gen- 

 eral 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 diverg- 

 ent 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 se- 

 quence of eruptions which those assumed conditions would give rise to con- 

 forms to the sequence which we find in nature. Let us, then, examine these 



