Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 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
Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 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 s
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