The country in which Edinburgh is situated has no great elevation above the level of the sea, presenting a gently undulating surface, except where hills of igneous origin, in groups, or perfectly insulated, rise abruptly through the strata, which consist of the sandstones and shales of the coal-formation, with occasional beds of limestone, which they overlie, and this country is more or less covered by old and new alluvial deposits.

The views suggested by these hills to the penetrating genius of Hutton, who may be justly considered the founder of modern geology, first led to the knowledge of the true nature and origin of the trap-rocks. Their analogy to those produced by existing volcanoes, and the phenomena observed in their relations with the secondary strata, leave no doubt as to their having been poured out from the interior of the earth in a fluid or viscid state, through fissures in the strata occasioned by subterranean convulsions—not, however, in the open air, like currents of lava from recent craters, but in sheets or masses at the bottom of the sea, their cooling and consolidation having evidently been slow and gradual, under great pressure, such as might be produced by a large volume of superincumbent water, as was ably illus trated by the experiments of the late Sir James Hall; or by their having been originally formed as dykes, at considerable depths, either below or among the strata.