portion of the earth's crust, however, we mark how not merely individuals have perished, but whole genera and species; but of these the remains are still before us in the rocks; we can study their forms, and, from a comparison with recent species and genera, can arrive at some idea of their nature and functions. In this way, we are able to picture the various conditions of the earth when these organisms lived in succession upon its surface. Yet, we may readily conjecture, that in ancient eras many tribes and genera of plants and animals lived for ages, and then passed away without leaving any record of their existence. Many circumstances might concur to prevent the preservation of their remains. The species of the Hebrides were preserved in the usual manner, but the cemetery in which their remains were entombed has been washed away, and they can be seen nowhere else. It is as if on some isolated country there had lived a race of men, tall Patagonians, or swarthy Hottentots, or diminutive Laplanders, with a civilisation of their own; owing to some change of climate the race gradually dwindled down until it died out; eventually, too, the land settled down beneath the sea with all its ruined cities and villages, which, as they reached in succession the level of the waves, were torn up and dispersed, and other races at last voyaged over the site of that old land, dreaming not, that in bygone years fellow-mortals of an extinct type had pastured their herds where now there rolled a widespread sea. [Footnote 15: _Quart. Jour. Geol. Soc._, vol. xiv. p. 26.] But to return. We have seen that the long-continued action of the sea has been sufficient to breach and waste away the existing coast-line of western Scotland. When, therefore, such results are produced by so ordinary a cause, need we go to seek the agency of great debacles to explain the denudation of other parts of the country? It is known that at great depths currents have little effect upon the rocks which they traverse, and that their action is greater as it nears the surface. To account for the phenomena of crag and tail, and the general denudation of the country, we may suppose the land to have been often submerged and re-elevated. As hill after hill rose towards or sank below the sea-level, it would be assailed by a strong current that flowed from the west and north-west, until, in its slow upward or downward progress, it got beyond the reach of the denuding agencies. In this way the general contour of the land would be greatly though very gradually changed. Hills of sandstone, or other material of feeble resistance, would be swept away, the harder trap-rocks would stand up bared of the strata which once covered them, deep hollows would be excavated in front of all the more prominent eminences, and long declivities would be left behind them.--(See Fig. 2.) If my reader has ever visited the channel of a mountain-torrent-- "Imbres Quern super notas aluere ripas"--