As on former occasions, I shall take this opportunity of stating that the "Manual" is not an epitome of the "Principles of Geology," nor intended as introductory to that work. So much confusion has arisen on this subject, that it is desirable to explain fully the different ground occupied by the two publications. The first five editions of the "Principles" comprised a 4th book, in which some account was given of systematic geology, and in which the principal rocks composing the earth's crust and their organic remains were described. In subsequent editions this book was omitted, it having been expanded, in 1838, into a separate treatise called the "Elements of Geology," first re-edited in 1842, and again recast and enlarged in 1851, and entitled "A Manual of Elementary Geology." Although the subjects of both treatises relate to geology, as their titles imply, their scope is very different; the "Principles" containing a view of the _modern_ changes of the earth and its inhabitants, while the "Manual" relates to the monuments of _ancient_ changes. In separating the one from the other, I have endeavoured to render each complete in itself, and independent; but if asked by a student which he should read first, I would recommend him to begin with the "Principles," as he may then proceed from the known to the unknown, and be provided beforehand with a key for interpreting the ancient phenomena, whether of the organic or inorganic world, by reference to changes now in progress. Owing to the former incorporation of the two subjects in one work, and the supposed identity of their subject matter, it may be useful to give here a brief abstract of the contents of the "Principles," for the sake of comparison. _Abstract of the "Principles of Geology," Eighth Edition._ BOOK I. 1. Historical sketch of the early progress of geology, chaps. i. to iv. 2. Circumstances which combined to make the first cultivators of the science regard the former course of nature as different from the present, and the former changes of the earth's surface as the effects of agents different in kind and degree from those now acting, chap. v. 3. Whether the former variations in climate established by geology are explicable by reference to existing causes, chaps. vi. to viii. 4. Theory of the progressive development of organic life in former ages, and the introduction of man into the earth, chap. ix. 5. Supposed former intensity of aqueous and igneous causes considered, chaps. x. and xi. 6. How far the older rocks differ in texture from those now forming, chap. xii. 7. Supposed alternate periods of repose and disorder, chap. xiii. BOOK II. CHANGES NOW IN PROGRESS IN THE INORGANIC WORLD. 8. Aqueous causes now in action: Floods--Rivers--Carrying power of ice--Springs and their deposits--Deltas--Waste of cliffs and strata produced by marine currents: chaps. xiv. to xxii.