I. THE STRUCTURE of the Sidlaw and Ochil ranges, from the amorphous character of the rocks, furnishes little or no room for geological description. These nearly parallel chains of hills, separated only by an interval of from two to five miles, and forming the lower water-shed of the basin of the Tay, consist of the various members of the trap family usually denominated whinstone, and whose structure is very different, upon a glance, from that of the sandstones and other sedimentary deposits we have been considering. This class of rocks have all a tendency, in mineralogical phraseology, to a spathose structure, and discover at least the rudiments of crystallization: there is no lamination in their internal texture, and the lines of stratification which they sometimes exhibit are assumed, or impressed by the previously consolidated strata among which they have been injected. They are not lavas, which are sub-aërial products, nor are they aqueous formations, whose materials have been deposited in water. These rocks are the results of igneous fusion deep under the crust of the earth, poured over the bottom of the sea, and protruded into the diversified dome-shaped forms which they generally present. Trap-rock consists of several varieties, as porphyry, clinkstone, compact feldspar, amygdaloid, greenstone, and basalt. These all pass into each other by insensible gradations, often forming one continuous mass, for the most part composed of the same ingredients, and have in consequence been regarded by geologists as belonging to one group, produced under similar circumstances, and elevated at intervals about the same period. The porphyritic structure prevails generally in both chains, and “porphyry has the peculiarity of being rarely found in any but the primary strata: it seems to be the whinstone of the Old World, or at least that which is of highest antiquity in the present.”[2] But not only are both ranges characterized by the same qualities and texture of rocks in hand-specimens, one hill answering to another; they also preserve the same general features of outline, and the same relations to the disrupted sandstones among which they have been injected. The highest point, for instance, in the eastern division of the Ochils, is Norman’s Law, attaining an elevation of nearly one thousand feet above the level of the sea: in its uprise the mass has brought along with it the lower beds of the gray sandstone, which flank its northern and eastern sides, within three hundred feet of the summit. To the north and west of Dundee the highest points of the Sidlaws are encompassed in like manner with their analogous beds of the gray rock. And so in every locality, whether along their base lines, or among the numerous ravines and valleys that intersect their cultivated slopes, the strata may be seen cropping out, bearing testimony to the convulsive movements to which they have been exposed, and the altered positions they have in consequence assumed.