At last, however, this condition of things came to be modified. The direction of the river channel along some part of its course had varied, or some analogous change had taken place, so that muddy sediment transported from the land sank down amid the cyprides at the bottom. In circumstances so uncongenial these tiny denizens of the estuary diminished in numbers until the silt and sand came down so rapidly and in such abundance that they eventually died out. Alluvial matter still darkened the water and covered the river-bottom, enveloping now the fronds of a delicate fern that had waved along the margin of some sequestered lake far inland, anon a seed-cone that had been shaken by the breeze from the spiky branches of some tall club-moss. Among these muddy beds occur numerous coprolites and fish-scales, along with cypriscases and a few shells of unio (?), showing that though the cyprides were decreasing, the water still presented the old estuary conditions and still swarmed with life. Eventually there came other changes in the direction or rapidity of river currents, and the accumulations of mud and silt were succeeded by a long protracted deposition of yellow sand, now forming the sandstone of Straiton. It enclosed many stems of stigmaria, lepidodendron, &c., and in certain limited areas these plants matted together in such quantities that their remains now form thin irregular seams of coal. It would appear, therefore, that notwithstanding these changes in the matter transported and deposited at the locality in question, the estuary character of the locality remained essentially the same. The sand was at length replaced by fresh accumulations of mud and sandy silt, which went to form the beds of shale and shaly sandstone now found above the Straiton rock. When in the course of many long centuries a depth of strata amounting to fully 300 feet had been amassed, the area of Mid-Lothian underwent a total change. Owing to a depression of the earth's crust, that seems to have been general over the whole of central Scotland, the estuary in which the Burdiehouse limestone and superincumbent strata were deposited became open sea. As the evidence of this change rests solely on the character of the imbedded organic remains, we shall pursue our induction by examining the beds somewhat in detail.