290 



LYELL'S ELEMENTS OF GEOLOGY. 



Granite altering Silurian Strata Most ancient Granites. 



Fig. 292. 



Silurian strata. 



Gneiss. Granite. Gneiss. 



Granite sending veins into Silurian strata and Oneiss, — Christiania, JVorway. 



pebbles of gneiss have been found in some of the transition 

 strata. Between the origin, therefore, of the gneiss and the gra- 

 nite there intervened, first, the period when the strata of gneiss 

 were incHned ; secondly, the period when they were denuded ; 

 thirdly, the period of the deposition of the transition deposits. 

 Yet the granite produced, after this long interval, is often so inti- 

 mately 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, tortu- 

 ous veins of granite pass freely through gneiss, ending some- 

 times in threads, as if the older rock had offered no resistance 

 to their passage. It seems necessary, therefore, to conceive that 

 the gneiss was softened and more or less melted when penetrated 

 by the granite. But had such junctions alone been visible, and 

 had we not learnt, 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 cha- 

 racter, 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 primary, or whether 

 they may not belong to some secondary or tertiary period. 



Most ancient 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 p. 23.) 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 depo- 

 sits. Could we discover some Lower Cambrian strata resting 

 immediately on granite, there being no alterations at . the point 

 of contact, nor any intersecting granitic veins, we might then 

 affirm the plutonic rock to have originated before the oldest 

 known fossiliferous strata. Still it would be presumptuous to 



