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THE OLD RED SANDSTONE.

■would have exhibited only tertiary depositions. In all theselines, whether of mountains, lakes, friths, or formations, thereis an approximation to parallelism with the line of the greatCaledonian Yalley,—proofs that the upheaving agency frombeneath must have acted in this direction from some unknowncause, during all the immensely extended term of its opera-tions, and along the entire length of the island. It is a factnot unworthy of remark, that the profound depths of Loch Ness undulated in strange sympathy with the reeling towersand crashing walls of Lisbon during the great earthquakeof 1755; and that the impulse, true to its ancient direction,sent the waves in huge furrows to the north-east and the south-west.

The north-eastern portion of this rectilinear wall or chainruns for about thirty miles through an Old Red Sandstonedistrict. The materials which compose it are as unlike thoseof the plain out of which it arises, as the materials of a stone-dyke running half-way into a field are unlike the vegetablemould which forms the field’s surface. The ridge itself is ofa granitic texture,—a true gneiss. At its base we find onlyconglomerates, sandstones, shales, and stratified clays, andthese lying against it in very high angles. Hence the geo-logical interest of this lower portion of the wall. As hasbeen shrewdly remarked by Mr Murchison,* in one of hisearlier papers, the gneiss seems to have been forced throughthe sandstone from beneath in a solid, not a fluid form ; and,as the ridge atop is a narrow one, and the sides remarkablyabrupt,—an excellent wedge both in consistency and form,—instead of having acted on the surrounding depositions, asmost of the south-country traps have done that have merelyissued from a vent and overlaid the upper strata, it has tornup the entire formation from the very bottom. Imagine alarge wedge forced from below through a sheet of thick ice* See “ Transactions of the London Geological Society ” for 1828, p. 354.