AGE OF THE GRANITOID AND PORPHYRITIC ROCKS. 95 



two eruptions of the same epoch may differ from each other without calling 

 for a distinction in their classification ; but the textural differences which 

 we now observe are due to the different conditions under which similar or 

 sensibly identical magmas have solidified. The granites have solidified 

 probably at great depths in the earth and under enormous statical pressure, 

 while volcanic rocks have solidified at the surface. Porphyries, which 

 usually occur in dikes or in intrusive masses, have solidified at intermedi- 

 ate horizons, though under conditions probably more nearly approaching 

 those of volcanic than of granitoid rocks. The Palaeozoic and Archaean 

 ages may have had their volcanic rocks, differing in no assignable respect 

 from those of recent date, and upon a scale as grand and equally varied, 

 but denudation has dissipated them. The granitoid rocks now exposed 

 to our view have been brought to the light of day only by an enormous 

 erosion, which has removed the thousands of feet of strata beneath which 

 they received their present texture. 



This explanation is fortunately capable of a test by comparison with 

 the facts presented by the rocks themselves, and though all the facts have 

 not been collected and studied in this light, yet our knowledge of their 

 general scope and bearing is considerable, and my belief is that they fairly 

 sustain the theory. The granites and syenites are almost invariably foimd 

 in localities where denudation has proceeded through a long series of 

 epochs and has been vast in amount* They are usually associated with 

 metamorphic rocks which have been laid bare by the removal of great 

 masses of superincumbent strata. They are not often found as interjected 

 beds in unaltered or little altered Palaeozoic or Mesozoic strata ; much less 

 as contemporaneous flows. The eruptive syenites and granites, therefore, 

 harmonize with the theory. 



The diorites and diabases have a different mode of occurrence. The 

 diorites, so far as known, are believed to be almost invariably intrusive,! 

 either in the form of dikes or intercalary between sedimentary beds. The 

 same also appears to be true of those diabases which possess an unquestion- 

 able granitoid texture. There are, indeed, many rocks to which the name 



* It would bo impracticable here to enter into a full discussion of particular cases without pro- 

 tracting the discussion indefinitely. The statement will, I think, be generally admitted. 

 t.Iukes and Geiko, Manual of Geology. 



