The Christiania granite alluded to above was at one time thought to have belonged to the Silurian period. But, in 1813, Von Buch announced that the strata in question consisted of limestones containing orthoceratites and trilobites; the shales and limestone being only penetrated by granite-veins, and altered for a considerable distance from the point of contact.[15] The same granite is found to penetrate the ancient gneiss of the country on which the fossiliferous beds rest--unconformably, as the geologists say; that is, they rest on the edges of the gneiss, from which other stratified deposits had been washed away, leaving the gneiss denuded before the sedimentary beds were deposited. “Between the origin, therefore, of the gneiss and the granite,”[16] says Lyell, “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 the 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, ending sometimes in threads, as if the older rock had offered no resistance to their passage.” From this example Sir Charles concludes that it is impossible to conjecture whether certain granites, which send veins into gneiss and other metamorphic rocks, have been so injected while the gneiss was scarcely solidified, or at some time during the Secondary or Tertiary period. As it is, no single mass of granite can be pointed out more ancient than the oldest known fossiliferous deposits; no Lower Cambrian stratum is known to rest immediately on granite; no pebbles of granite are found in the conglomerates of the Lower Cambrian. On the contrary, granite is usually found, as in the case of Dartmoor, in immediate contact with primary formations, with every sign of elevation subsequent to their deposition. Porphyritic pebbles are found in the Coal-measures; porpyhries continue during the Triassic period; since, in some parts of Germany, veins of porphyry are found traversing the New Red Sandstone, or _grès bigarré_ of French geologists. Syenites have especially reacted upon the Silurian deposits and other old sedimentary rocks, up to those of the Lower Carboniferous period. [15] “Elements of Geology,” p. 717. [16] Ibid, p. 718. The term porphyry is usually applied to a rock with a paste or base of compact felspar, in which felspathic crystals of various sizes assume their natural form. The variety of their mineralogical characters, the admirable polish which can be given to them, and which renders them eminently useful for ornamentation, give to the porphyries an artistic and industrial importance, which would be greatly enhanced if the difficulty of working such a hard material did not render the price so high.