Since the first appearance of land in the region now occupied by North America, sedimentary rocks have been in process of formation, and in this way the growth of the continent, with the aid of movements in the earth's crust, has been produced. The superficial extent of the sedimentary beds in North America is very great, as is indicated on the map referred to above. By far the larger portion of the surface of the continent is underlain by them. Their thickness varies from place to place, but probably reaches a maximum in the Appalachian region, where a depth of some 40,000 feet has been measured. Throughout the continental basin their depth is in general from 3,000 to 4,000 feet. In the Pacific mountains their thickness embraces tens of thousands of feet, and the same is true in Mexico, Cuba, and Jamaica. These sedimentary rocks contain fossils which, with comparatively few exceptions, show that they were deposited in the ocean; thus sustaining in an important manner the conclusion already presented in reference to the growth of the continent. Great as is the area of the sedimentary beds at the present time, it does not show the entire extent to which what is now land has at some time been submerged beneath the sea. In certain broad regions, sedimentary beds which formerly existed have been removed by erosion; in other extensive areas they are covered by volcanic rocks, and in still other portions of the continent, embracing thousands of square miles, they have been metamorphosed and their original characteristics obliterated. The system of classification of the sedimentary beds that has been adopted, as is well known, is based on the relative age of the formations, determined primarily by the occurrence of one formation above another, in regions where but moderate disturbances in position have occurred. Many of the stratified rocks contain fossils--that is, records of the life of the time they were deposited, and after the order of succession of a large number of formations has been ascertained, the life records they contain may be used as a means of determining the age of a newly discovered terrane. By grouping the information obtained from the study of the vertical sequence of the formations in many regions, and also the records of life contained in them, a composite geological column has been constructed which shows the relative age of all known formations. The larger divisions of such a scheme of classification are world-wide in their application, but the smaller divisions are usually of restricted geographical extent.