Such is the economy of nature around us now, and that such will continue to remain the condition of things in the future, we can affirm with probability from a consideration of the history of the past. The geologist can point to masses of rock several miles in thickness, and occupying a large area of the globe, formed entirely of the worn debris of pre-existing formations. The very oldest rocks with which he is acquainted are made up of hardened sediment, pointing to the existence of some land, even at that early period, worn down by rivers or wasted by the sea. During all the subsequent ages the same principles were at work, and now well-nigh the only evidence of the geological periods is to be gathered from the layers of sediment that successively settled down at the sea-bottom. The records which it is the task of the geologist to decipher, are for the most part written in sand and mud--the deposits of the ocean, for in by far the larger number of formations into which the stratified part of the earth's crust has been divided, and which form his only guide to the history of the past, he can detect no trace of land. Hill and valley have alike disappeared, and the character of their scenery and inhabitants he can often but dimly conjecture from the nature of the sediment and of the drifted terrestrial relics that may chance to be found among strata wholly marine. CHAPTER X. The structure of the stratified part of the earth's crust conveniently studied by the examination of a single formation--A coal-field selected for this purpose--Illustration of the principles necessary to such an investigation--The antiquities of a country of value in compiling its pre-historic annals--Geological antiquities equally valuable and more satisfactorily arranged--Order of superposition of stratified formations--Each formation contains its own suite of organic remains--The age of the boulder defined by this test from fossils--Each formation as a rule shades into the adjacent ones--Mineral substances chiefly composing the stratified rocks few in number--Not of much value in themselves as a test of age--The Mid-Lothian coal-basin--Its subdivisions--The limestone of Burdiehouse--Its fossil remains--Its probable origin--Carboniferous limestone series of Mid-Lothian--Its relation to that of England--Its organic remains totally different from those of Burdiehouse--Structure and scenery of Roman Camp Hill--Its quarries of the mountain limestone--Fossils of these quarries indicative of an ancient ocean-bed--Origin of the limestones--Similar formations still in progress--Coral-reefs and their calcareous silt--Sunset among the old quarries of Roman Camp Hill.