Such is the theory of Geology as expounded at the present day by its most able and popular advocates. We have passed over a multitude of minor details that we might not weary our readers, and we have kept aloof from disputed points that we might not get entangled in a purely scientific controversy. Our object has simply been to gather together into a systematic form those more general conclusions which, however startling they may seem to practical men of the world, and even to many of those whose minds have been accustomed to the pursuit of science in other departments, are nevertheless regarded as certain by all who have devoted their lives to the study of Geology. It now remains to investigate the facts on which these conclusions are based, and to consider the line of argument by which so many able and earnest men have been led to accept them. In this vast field of inquiry we shall chiefly direct our attention to those points that bear upon the Antiquity of the Earth; and in attempting to bring home to our readers the nature and the force of Geological reasoning, we shall confine ourselves altogether to simple and familiar illustrations. [Illustration] [Illustration] _CHAPTER II._ THEORY OF DENUDATION ILLUSTRATED BY FACTS. _Principle of reasoning common to all the physical sciences--This principle applicable to Geology--Carbonic acid an agent of denudation--Vast quantity of lime dissolved by the waters of the Rhine and borne away to the German ocean--Disintegration of rocks by frost--Professor Tyndall on the Matterhorn--Running water--Its erosive power--An active and unceasing agent of denudation--Mineral sediment carried out to sea by the Ganges and other great rivers--Solid rocks undermined and worn away--Falls of the Clyde at Lanark--Excavating power of rivers in Auvergne and Sicily--Falls of Niagara--Transporting power of running water--Floods in Scotland--Inundation in the valley of Bagnes in Switzerland._