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may resemble those of the sometimes-underlying Potomac, or Tuscaloosa series. Again, the physical features of the whole formation are often reproduced in the overlying Columbia formation. Although the Lafayette is remarkably persistent in its characters over the enormous area, yet care must be exercised in its study. In exposed sections, the surfaces become case-hardened, and stand as vertical walls, on which often the shades of ferruginous oxidation can be seen. The subjacent formations give rise to local variations in the amount of sand, clay, or calcareous matter, which is particularly shown in the agricultural features. This formation once covered the entire coastal plain of both the Atlantic and Gulf margins from Maryland to Mexico, and extended up the Mississippi embayment as far as the mouth of the Ohio, covering a belt extending from the sea margin 50 to 200, or even 500 miles into the interior of the continent. Often, the deposits form only a thin mantle, and away from the valleys ten or twenty feet may be regarded as an average thickness. In the valleys, the accumulations reach 120 feet, and toward the mouth of the Mississippi, 200 feet or more. But the formation has been degraded to an enormous extent by erosion, which has removed it from broad areas, leaving only patches to mark its former extension.

In an introductory chapter, the author has given us an excellent description of the physiography of the coastal plain and of the various geological series in contact with the Lafayette formation. On the Atlantic border, the interior of the coastal plain is sharply defined by the margin of the Piedmont plateau, generally characterized by Archean rocks. This margin is the "fall line," or location of the last great rapids in the descent of the rivers to the sea. Below this line, the streams, which generally cross the plains, are more or less navigable. The interior margin of the Gulf coastal plain is less sharply defined, as it trends across the termination of many different formations of varying characteristics. This same coastal plain extends seaward to the margin of the continental shelf, which is now submerged and extends far seaward of the present coast.

The geology of this plain presents a varied study. Generally speaking, the Potomac (or Tuscaloosa) or later Cretaceous deposits form the interior margin of the belt. This basement is succeeded by many stages of the higher Cretaceous, Eocene, and Miocene accumulations, although the succession is not everywhere complete. No marine fossils higher than the middle Miocene are known on the