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Where different genera are intermingled, there often exists a systematic relation between the series of organic forms and the superposition of the formations; and it has been remarked that the association of certain families and species follows a regular law in the superimposed strata of which the whole constitutes one formation. It has been found that the waters in the most distant parts of the globe were inhabited at the same epochs by testaceous animals corresponding, at least in generic character, with European fossils.

Strata defined by their fossil contents, or by the fragments of other rocks which they include, form a geological horizon by which the geologist may recognise his position, and obtain safe conclusions in regard to the identity or relative antiquity of formations, the periodical repetition of certain strata—their parallelism—or their entire suppression. If we would thus comprehend in its greatest simplicity the general type of the sedentary formations, we find in proceeding successively from below upwards: (1) The Transition group, including the Silurian and Devonian (Old Red Sandstone) systems; (2) the Lower Trias, comprising mountain limestone, the coal measures, the lower new red sandstone, and the magnesian limestone; (3) the Upper Trias, composing the bunter, or variegated sandstone, the muschelkalk, and the Keuper sandstone; (4) the Oolitic, or Jurassic series, including Lias; (5) the Cretaceous series; (6) the Tertiary group, as represented in its three stages by the calcaire grossier and other beds of the Paris basin, the lignites, or brown coal of Germany, and the sub-Apennine group of Italy.

To these succeed transported soils (alluvium), containing the gigantic bones of ancient mammalia, such as the mastodons, the dinotherium, and the megatheroid animals, among which is the mylodon of Owen, an animal upwards of eleven feet in length, allied to the sloth. Associated with these extinct species are found the fossil remains of animals still living: elephants, rhinoceroses, oxen, horses, and deer. Near Bogota, at an elevation of 8,200 French feet above the level of the sea, there is a field filled with the bones of mastodon (Campo de Gigantes), in which I have had careful excavations made. The bones found on the table-lands of Mexico belong to the true elephants of extinct species. The minor range of the Himalaya, the Sewalik hills, contain, besides numerous mastodons, the sivatherium and the gigantic land-tortoise (Colossochelys), more than twelve feet in length and six in height, as well as remains belonging to still existing species of elephants, rhinoceroses, and giraffes. It is worthy of notice that these fossils are found in a zone which enjoys the tropical climate supposed to have prevailed at the period of the mastodons.