386 
persistency of the elementary forms is equally remarkable, 
resisting as it does all the forms of force that can be brought 
to bear upon it. 
10. If we begin at the lowest rocks, we are at once met by 
one of those problems, the solution of which still remains a 
mystery, — I mean the formation of granite. Occurring as it 
does among the very earliest rocks, so many of which bear the 
most evident traces of fusion, it was for long taken for granted 
that this singular formation also w r as of the nature of a lava, 
and that it resulted from the cooling of a melted mass. 
11. The separation and rearrangement of its constituents into 
the well-known definite crystals of felspar, mica, and quartz, 
that make up granite, by gradual cooling, is conceivable, though 
we are quite unable to repeat the process by again fusing and 
cooling the mass ; but this hypothesis is shown to be untenable, 
bv the curious fact that the crystals of felspar are found to be 
embedded in those of mica and quartz ; felspar, however, is the 
most fusible of the three constituents ; and therefore, if the 
crystallization was caused by the cooling of a fused mass, must 
have formed last, in which case the quartz and mica would have 
been embedded in felspar. Finally, we find veins of granite 
running through rocks which do not bear, as we should expect, 
signs of the tremendous heat to which they must have been 
exposed to allow the granite to remain fluid while penetrating 
into the vein, and this point should specially be noted, as the 
structure of granite could not possibly be produced except by 
slow cooling. 
12. All these considerations lead us to the conclusion that we 
must look to some other cause for the origin of granite ; and 
though we may vaguely guess that it may probably have been 
formed b} r the combined effects of heat and pressure in the 
presence of Avater, the guess is but a confession of our igno- 
rance of the conditions of its formation, and still more of the 
causes that brought about those conditions. 
13. We are thus, in the very first step of the inquiry, brought 
face to face with a problem well suited to impress us with the 
vastness of those forces, that, guided by some directing cause, 
have so wonderfully wrought upon the face of our earth. 
14. There is somewhat less difficulty in understanding the 
formation of the other igneous rocks, though we cannot but be 
struck by the diversity of composition, which marks a selective 
power in nature, of which \vc can form but little idea. Here, 
too, in this simpler question we find curious difficulties ; the 
