36-t AGE OF PLUTONIC ROCKS. [Ch. XXV. 



contrary, that it has been heated, and still continues for cen- 

 turies, or thousands of years, at a red heat, the vein may 

 acquire a highly crystalline texture. 



The great pressure of a superincumbent mass, and exclusion 

 from contact with air or water, are probably the usual con- 

 ditions necessary to produce the granitic texture; but the 

 same may sometimes be superinduced at a slighter distance 

 from the surface by slow refrigeration, when additional sup- 

 plies of heat check, from time to time, the cooling process and 

 cause it to be indefinitely protracted. 



If, for the reasons above alluded to, we conceive it probable 

 that plutonic rocks have originated in the nether parts of the 

 earth's crust, as often as the volcanic have been generated at 

 the surface, we may imagine that no small quantity of the 

 former class has been forming in the recent epoch, since we 

 suppose that about 2000 volcanic eruptions may occur in the 

 course of every century, either above the waters of the sea or 

 beneath them *. 



We may also infer, that during each preceding period, 

 whether tertiary or secondary, there have been granites and 

 granitiform rocks generated, because we have already dis- 

 covered the monuments of ancient volcanic eruptions at almost 

 every period. 



In the next chapter we shall endeavour to show, that in 

 consequence of the great depths at which the plutonic rocks 

 usually originate, and the manner in which they are associated 

 with the older sedimentary strata of each district, it is rarely 

 possible to determine with exactness their relative age. Yet 

 there is reason to believe that the greater portion of the plutonic 

 formations now visible are of higher antiquity than the oldest 

 secondary strata. We shall also endeavour to point out, that 

 this opinion is by no means inconsistent with the theory that 

 equal quantities of granite may have been produced in succes- 

 sion, during equal 2^criods of time, from the earliest to the most 

 modern epochs. 



* Sec vol. i. cliap. xxii. 



