But whence these abrupt, precipitous hills that stud the landscape, and form, in the immediate neighborhood of the city, its more striking features? They belong--to return to the illustration of the twice-frozen lake--to the middle period of thaw, when the ice broke up; and, as they are composed chiefly of matter ejected from the abyss, might have characterized equally any of the other formations. Their very striking forms, however, illustrate happily the operations of the great agencies on which, in the secondary and transition deposits, all the peculiarities of scenery depend. The molten matter from beneath seems to have been injected, in the first instance, through rents and fissures among the carboniferous shales and sandstones of the district, where it lay cooling in its subterranean matrices, in beds and dikes, like metal in the moulds of the founder; and the places which if occupied must have been indicated on the surface but by curves and swellings of the strata. The denuding power then came into operation in the form of tides and currents, and ground down the superincumbent rocks. The injected masses, now cooled and hardened, were laid bare; and the softer framework of the moulds in which they had been cast was washed from their summits and sides, except where long ridges remained attached to them in the lines of the current, as if to indicate the direction in which they had broken its force. Every larger stone in a water-course, after the torrent fed by a thunder shower has just subsided, shows, on the same principle, its trail of sand and shingle piled up behind it. The outlines of the landscape were modified yet further by the yielding character of the basement of sandstone or shale on which the Plutonic beds so often rest. The basement crumbled away as the tides and waves broke against it. The injected beds above, undermined in the process, and with a vertical cleavage, induced by their columnar tendency, fell down in masses that left a front perpendicular as a wall. Each bed came thus to present its own upright line of precipice; and hence--when they rise bed above bed, as often occurs--the stair-like outline of hill to which the trap rocks owe their name; hence the outline of the Dalmahoy Crags, for instance, and of the southern and western front of Salisbury Crags.