1897-98]- HIGH PLATEAl AND SUBMARINE ANTILLEAN VALLEYS. 361 



these flood-plains are bounded by high bluffs of soft or incoherent rocks 

 of Cretaceous and early Tertiary ages, and later superficial deposits, in 

 contrast to the hard Paleozoic formations of the upper section of the 

 valley. The difference of width of the upper and lower portions of the 

 valley is mostly due to the rainfall acting upon the more readily yielding 

 strata during a long period of base level of erosion. The lower part of 

 the buried valley reaches to the great depth of about i,ooo feet below 

 the Gulf of Mexico.* If the region should further subsidef by 300 feet, 

 the river would become an estuar}' like that of the St. Lawrence. 



The Colorado River of the West forms another type of valleys. It 

 flows from elevated plateaus, some of which have altitudes of 8,000 or 

 10,000 feet above the sea. From Echo Cliff, the Marble canyon extends 

 sixty-six miles to the mouth of the Little Colorado River, below which 

 the gorge of the Grand canyon reaches 195 miles to Grand Wash, but 

 the canyon is twenty-five miles longer. The gradient of the river is 

 commonly between seven and eight feet per mile, although reduced in 

 parts to five feet, or increased to twelve feet in others, and locally it is 

 nowhere more than twenty-five feet. The mean slope is accordingly 8.5 

 feet per mile. (These slopes are illustrated in figure 2, page 364). The 

 inner canyon may have a breadth equal to the depth, or from 3,500 to 

 4,000 feet, but the outer canyon has a width of from five to over twelve 

 miles, with bounding escarpments 2,000 feet above the higher floor. 

 The outer valley suggests that the region was at a lower base level, 

 when the altitude could not have been at more than a slight elevation 

 above the sea, like that of the modern Mississippi valley. 



The gorge of the Niagara River is seven miles long, and although the 

 volume of water is very large, the mean declivity is about sixteen feet 

 per mile ; but locally there are great rapids and eddies. 



Equalh' important, with the study of the slopes of the great valleys and 

 rivers, is that of the gradients of their short tributaries. These are often 

 hardly more than stupendous washouts, forming great amphitheatres. 

 The gradients of these short valleys may be from 400 to 600 feet per mile 

 for a few miles, but this is greater near its head, and less along its lower 

 portions. The declivity is never less than 200 feet per mile (Dutton:^). 



• The depth has long been known to exceed 600 feet, but lately Mr. E. L. Corthell has found by boring-s 

 that it reaches to about 1,000 feet. 



t Mr. Corthell has also found that the region about New Orleans is sinking at the rate of five feet a 

 century. "Geographical Development of the Lower Mississippi." Communicated to the British Association, 

 Toronto, 1897. 



J The measurements were taken from the " Tertiarv Ilistorv of the Grand Canvou District " (of the 

 Colorado of the West), by C. E. Dutton (1882). 



