This account, as given by geologists, of the origin of the mountain limestone, is rendered not only probable, but almost certain, by the manner in which we find these little insects, the coral-builders, constructing their piles of masonry at the present day. For example, certain species of polyps, of solitary habits, work alone, each rearing a single stem or stalk, from which others project; then more stems are produced, until, upon the completion of the whole, there results one of those beautiful arborescent structures so much prized as ornaments for cabinets and drawing-rooms. Some, again, attach themselves to the loose stones, upon which they form their little tree or flower-top; others adhere to the solid rock, from which there springs a stony vegetation, rivaling often, in variety, luxuriance, and brilliancy, the most showy vegetable productions of tropical climes. But a certain class are gregarious, and will only work in company. Myriads of these inhabit the Pacific, constructing entire islands, and throwing up mighty barriers of rock, and threading over vast areas of the sea with inosculating lines of coral reef. The calcareous accumulation, known as the Great Barrier Reef, extends for about a thousand miles in length, by about thirty in mean breadth, filling up, with its various reticulations, the whole intermediate space betwixt the coast of Australia and Bristow Island, off the coast of New Guinea. The works of these minute creatures thus occupy an area which may be roughly estimated at thirty thousand square miles; the different branches forming compartments of variable extent, which are divided into linear, outer, and inner reefs, and embracing within their ample folds the entire spoils of ocean living or floating in these parts.