726 



OLDEST GKANITE ROCKS. 



[Ch. XXXIY. 



tion of gneiss ; and the relations of the plutonic rock and the gneiss, 

 at their junction, are full of interest when we duly consider the wide 

 difference of epoch which must have separated their origin. 



The length of this interval of time is attested by the following 

 facts : The fossiliferous or Silurian beds rest uncorjformably upon the 

 truncated edges of the gneiss, the inclined strata of which had been 

 denuded before the sedimentary beds were superimposed (see fig. 753). 



ig. 753. 



Gneiss. Granite. Gneiss. 



Granite sending veins into Silurian strata and Gneiss. Christiania, Norway. 



The signs of denudation are twofold ; first, the surface of the gneiss is 

 seen occasionally, on the removal of the newer beds, containing organic 

 remains, to be worn and smoothed; secondly, pebbles of gneiss 

 have been found in some of these Silurian strata. Between the origin, 

 therefore, of the gneiss and the granite there intervened, first, the 

 period when the strata of gneiss were denuded ; secondly, the period 

 of the deposition of the Silurian deposits. Yet the granite produced 

 after this long interval is often so intimately blended with the ancient 

 gneiss, at the point of junction, that it is impossible to draw any other 

 than an arbitrary line of separation between them ; and where this is 

 not the case, tortuous veins of granite pass freely through gneiss, end- 

 ing sometimes in threads, as if the older rock had offered no resist- 

 ance to their passage. These appearances may probably be due to 

 hydrothermal action (see below, p. 740). I shall merely observe in 

 this place, that had such junctions alone been visible, and had we not 

 learned, from other sections, how long a period elapsed between the 

 consolidation of the gneiss and the injection of this granite, we might 

 have suspected that the gneiss was scarcely solidified, or had not yet 

 assumed its complete metamorphic character when invaded by the 

 plutonic rock. From this example we may learn how impossible it is 

 to conjecture whether certain granites in Scotland, and other countries, 

 which send veins into gneiss and other metamorphic rocks, are pri- 

 mary, or whether they may not belong to some secondary or tertiary 

 period. 



Oldest Granites. — It is not half a century since the doctrine was 

 very general that all granitic rocks were primitive, that is to say, that 

 they originated before the deposition of the first sedimentary strata, 

 and before the creation of organic beings (see above, p. 9). But so 

 greatly are our views now changed, that we find it no easy task to 

 point out a single mass of granite demonstrably more ancient than all 

 the known fossiliferous deposits. Could we discover some Lower 



