According to the natural division of the continent, North America begins about the 20th degree of north latitude, and terminates in the Arctic Ocean. It is longer than South America, but the irregularity of its outline renders it impossible to estimate its area. Its greatest length is about 3100 miles, and its breadth, at the widest part, is 3500 miles

The general structure of North America is still more simple than that of the southern part of the continent. The table-land of Mexico and the Rocky Mountains, which are the continuation of the high land of the Andes, run along the western side, but at a greater distance from the Pacific; and the immense plains to the east are divided longitudinally by the Alleghanny Mountains, which stretch from the Carolinas to the Gulf of St. Lawrence, parallel to the Atlantic, and at no great distance from it. Although the general direction of the two chains is from south to north, yet, as they maintain a degree of parallelism to the two coasts, they diverge towards the north, one inclining towards the north-west, and the other towards the north-east. The long narrow plain between the Atlantic and the Alleghannies is divided, throughout its length, by a line of cliffs not more than 200 or 300 feet above the Atlantic plain—the outcropping edge of the Second Terrace, or Atlantic Slope, whose rolling surface goes west to the foot of the mountains.