Leaving behind us the quarries of Carmylie, we descend the hill-side, and rise in the system as we lower our level and advance upon the sea. For a very considerable distance we find the rock covered up by a deep-red diluvial clay, largely charged with water-worn boulders, chiefly of the older primary rocks, and of the sandstone underneath. The soil on the higher grounds is moory and barren--a consequence, in great part, of a hard, ferruginous pan, which interposes like a paved floor between the diluvium and the upper mould, and which prevents the roots of the vegetation from striking downwards into the tenacious subsoil. From its impervious character, too, it has the effect of rendering the surface a bog for one half the year, and an arid, sun-baked waste for the other. It seems not improbable that the heaths which must have grown and decayed on these heights for many ages, may have been main agents in the formation of this pavement of barrenness. Of all plants, they are said to contain most iron. According to Fourcroy, a full twelfth part of the weight of oak, when dried, is owing to the presence of this almost universally diffused metal; and the proportion in our common heaths is still larger. It seems easy to conceive how that, as generation after generation withered on these heights, and were slowly resolved into a little mossy dust, the minute metallic particles which they had contained would be carried downwards by the rains through the lighter stratum of soil, till, reaching the impermeable platform of tenacious clay beneath, they would gradually accumulate there, and at length bind its upper layer, as is the nature of ferruginous oxide, into a continuous stony crust. Bog iron, and the clay ironstone, so abundant in the Coal Measures, and so extensively employed in our iron-works, seem to have owed their accumulation in layers and nodules to a somewhat similar process, through the agency of vegetation. But I digress.