4 FOURTH REPORT — 1834. 



geographical and geological boundary, commencing on the coast 

 of Massachusetts and running to Alabama. The boundary meant 

 is the eastern edge of a well exposed range of primary rocks, 

 which, from New Jersey as far south as North Carolina, forms a 

 nearly definite limit to the flowing up of the tide in the Atlantic 

 rivers. Between it and the ocean the country is throughout 

 low, flat, and sandy, while westward the rest of the plain rises 

 in gradually swelling undulations to the base of the blue ridge 

 or eastern chain of the Alleghanies. The rivers descend from 

 the mountains over this western portion of the tract, precipitate 

 themselves over the rocky boundary mentioned, either in falls 

 or long rapids, and emerge into the tide level to assume at once 

 a totally new character. South of North Carolina this line of 

 primary rocks leaves the tide and retires much nearer to the 

 mountains, though it still preserves its general featui-es, sepa- 

 rating the rolling and pictm-esque region of the older rocks from 

 the tertiarjr plains next the ocean ; and though the tide does not 

 any longer lave its base, as in Virginia, Maryland and Pennsyl- 

 vania, it still produces rapids and cataracts in the southern 

 rivers which cross it. Ranging for so very great a distance 

 with a remarkable uniformity of outline and height, on an 

 average between 200 and 300 feet above the tide, it consti- 

 tutes as admirable a geographical limit as it does a commercial 

 one. Nearly all the chief cities of the Atlantic States have 

 arisen upon this boundary, from the obvious motive of seek- 

 ing the head of navigation ; a striking example of the influence 

 of geological causes in distributing population and deciding the 

 political relations of an extensive country. Below this boundary 

 the aspect of the region is low and monotonous, the general 

 average elevation of the plain probably not exceeding 100 feet. 

 Its general width through the Middle and Southern States is 

 from 100 to 150 miles. As the tide enters this tract so exten- 

 sively, flowing, except in the more southern States, entirely 

 across it, a series of very abundant alluvial deposits occurs, dis- 

 tributed throughout. The surface is everywhere scooped down 

 from the general level to that of the tide by a multiplicity of 

 valleys and ravines, the larger of which receive innumerable 

 inlets and creeks, while the smaller contain marshes and allu- 

 vial meadows. The whole aspect of the barrier of primary rocks 

 forming the western limits of this plain forcibly suggests the 

 idea that at a rather lower level they once formed the Atlantic 

 shore, and that they exposed a long line of cliff's and hills of 

 gneiss to the fury of the ocean : a survey of the plain just de- 

 scribed as strongly suggests the idea that all of it has been lifted 

 from beneath the waves by a submarine force, and its surface 



