. Elements of geology. Geology. 290 LYELL'S ELEMENTS OF GEOLOGY. Granite altering Silurian Strata Most ancient Granites. Fig. 292. Silurian 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, afte
. Elements of geology. Geology. 290 LYELL'S ELEMENTS OF GEOLOGY. Granite altering Silurian Strata Most ancient Granites. Fig. 292. Silurian 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.) B
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