436 THE JOURNAL OF GEOLOGY. 



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 feat- 

 ures. 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, cover- 

 ing a belt extending from the sea ma'rgin 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 Atlan- 

 tic 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 vary- 

 ing characteristics. This same coastal plain extends seaward to the margin 

 of the continental shelf, which is now submerged and extends far sea- 

 ward 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 accumu- 

 lations, although the succession is not everywhere complete. No 

 marine fossils higher than the middle Miocene are known on the 



