Page:A Treatise on Geology, volume 1.djvu/167

 CHAP. VI. periods. In many instances the Silurian and old red strata are unconformedly situated with respect to one another, as in the subjoined section (fig. 4-2).

and the geological map of the country shows superficial unconformity of direction and dip of strata as in fig. 43.

The position of the secondary strata is discordant with respect to the primary, both in dip and direction; because these latter were disturbed from their original position by subterranean forces, and the bed of the sea upon which the secondary rocks were subsequently spread entirely altered in form. The unconformity, above exemplified, is the geological proof that the older strata had been disturbed previously to the formation of the newer; and the reason for thinking they had been in many cases actually raised into dry land, is the total absence of any later deposit upon them: the former is a most certain conclusion; the latter is frequently a highly probable inference.

In the British islands we have magnificent examples of these ancient disturbances. The range of the Grampian mountains from Aberdeen to Cantire, and indeed most of the Highlands, appear to have been uplifted at this early period, if not to the surface, yet so as to prevent any depositions upon them; though round the east and west coasts of Scotland, the south border of the Grampians, and in the great valley of the Caledonian canal, the old red sandstone rocks abound. It is