360 TRANSACTIONS OF THE CANADIAN INSTITUTE. [VOL. V. 



valleys date from the earlier Pleistocene epoch, even if in part they 

 should be of greater age, refilled with these last named deposits, which 

 were largely carried away by subsequent denudation during the epoch 

 of great elevation. 



In the original paper on the " Reconstruction of the Antillean Con 

 tinent," the declivities of the land valleys were scarcely compared with 

 those drowned beneath the sea. Indeed, in order to draw the analogy 

 it was necessary to make a study of the slopes of the valleys dissecting 

 the margins of the high table-lands of Mexico* and the Western States, 

 and such comparison is offered in the following pages. 



In the Appalachian mountains some of the valleys are from ten to 

 twenty miles across, occupying in places anticlines in the structure of 

 the formations, or extending over many anticlines and synclines alike, 

 or crossing the trends of the beds, for there the strata are usually much 

 folded or upturned. The valley of Lookout Creek varies from two to 

 four miles in width, and it is bounded by table-lands from 500 to 1,000 

 feet above it. Even at the divide between its head and that of Will's 

 Creek, flowing in the opposite direction, its breadth is more than two 

 miles, and the bounding escarpments 500 feet high. The average 

 declivity of the valley is over ten feet per mile. 



The Mississippi system is a representative of great streams flowing 

 over a continental plain, which is now so reduced in height above the 

 sea level, that there is comparatively little deepening of the channels, 

 and indeed, the valleys of all the larger branches are more or less 

 deeply silted over with river deposits. Above the mouth of the Ohio, 

 the Mississippi River flows through a valley from three to eleven miles 

 wide, bounded by rocky escarpments, commonly of limestone, rising 

 from 100 to 500 feet above the floor. The length of this reach is about 

 700 miles, although over 800 miles, if the winding of the valley be 

 more closely followed. The mean gradient of this section is six- 

 tenths of a foot per mile. From the mouth of the Ohio River to 

 the Gulf of Mexico, the direct distance is 550 miles, although the 

 river is twice as long. The mean slope of this portion of the 

 valley is five-tenths of a foot per mile, while that of the river is 

 only half as much. The Mississippi River now flows over a buried 

 valley, the floor of which is from 100 to 200 or 300 feet below its sur- 

 face, so that the declivity of the ancient channel below the mouth of the 

 Ohio is over one foot per mile. The river throughout this portion of its 

 course, wanders over alluvial flats from forty to eighty miles wide, and 



* " Great Chang-es of Level in Mexico and the Interoceanic Connections." Bull. Geol. Soc. Am., vol. 

 ix.. pp. 13-34 '897. 



