Now, though recent discoveries have shown that this rule has many exceptions, and that its too stringent adoption has been productive of some erroneous generalizations, yet if employed with due caution it is fraught with the most interesting results, and is the only certain basis of our knowledge respecting the appearance, continuance, and extinction, of the lost races of animals and plants, which were once denizens of our planet. [Sidenote: ROCKS AND STRATA.] In the "Wonders of Geology" will be found a comprehensive sketch of the composition and arrangement of the several formations or groups of strata; and a reference to that work will afford the student the necessary information on this branch of Geology. For the convenience of the general reader I subjoin a synoptical view of the characters and relations of the British fossiliferous deposits. The total thickness of the entire series of rocks within the scope of human examination, is estimated at from fifteen to twenty miles, reckoning from the summits of the highest mountains to the greatest depths hitherto penetrated; and as this vertical section scarcely amounts to 1/400th of the diameter of the globe, it is familiarly termed the Earth's crust. The substances of which the sedimentary strata are composed have been deposited by the action of water, and subsequently more or less modified in structure and composition by heat, and by electro-chemical forces. The superficial accumulations of water-worn detritus, consisting of gravel, boulders, sand, clay, &c. are termed _Drift_, or _Alluvial_ deposits. When the successive layers in which the sediments subsided are obvious, the deposits are said to be _stratified_; when the nature of the materials has been altered by igneous action or high temperature, but the lines of stratification are not wholly effaced, the rocks are denominated _metamorphic_ (_transformed_). When all traces of organic remains and of sedimentary deposition are lost, and the mass is crystalline, and composed of known products of igneous action, such rocks are named _plutonic_, as granite, sienite, trap, basalt, porphyry, and the like. Lastly, rocks resembling the lavas, scoria, and other substances emitted by burning mountains still in activity, are called _volcanic_.