i8 



POPULAR SCIENCE MONTHLY. 



that tliej can not be scaled. The depths vary from a few hundred 

 feet to -a couple of thousand feet, or even more. The same features 

 are found l^eneath the sea. 



On the Declivity of Land Valleys. — The declivity of the 

 greater land valleys crossing regions of continental extent is usually 

 very gentle — a foot per mile, more or less. Smaller valleys may 

 have a declivity of five or ten feet per mile. In the short amphi- 

 tlieaters, descending from the high plateaus, the gradients may be two 

 hundred or even five hundred feet or more per mile. The valleys dis- 

 secting recently elevated table-lands, such as those of Mexico, have 

 not uniform slopes, but the descent is characterized by a great series 

 of steps, the surface of each often appearing nearly level to the eye, 

 but with abrupt margins. The vertical heights of such steps vary 

 from five feet to even five hundred feet. The character of the slopes 

 is illustrated in Fig. 4. If stretches of several miles be taken, so 



Fig. 4. — Longitudinal section of a Mexican valley (above Atoyae), showing the descent in 



steps. 



many gradation plains occur that the mean declivity of the valley 

 may be from seventy-five to one hundred and fifty feet per mile. 

 Each of these represents pauses in the elevation of the region, while 

 the streams were flowing at levels so low that they could not further 

 deepen their channels, but were widening them out into broad val- 

 leys or plains — that is to say, the gradation plains represent base 

 levels of erosion. The valleys on the surface of the table-lands have 

 usually low gradients, which may be as small as those of rivers cross- 

 ing continental plains. Similar gradation steps are found in valleys 

 beneath the sea. 



Submarine Plateaus. — The low coastal plains of the South- 

 eastern States do not terminate at the seashore, but pass beyond, 

 forming shoals and banks, and eventually submarine plateaus, over- 

 looking the edge of the continent, which is fifteen miles eastward of 

 Cape Hatteras, but three hundred miles distant from the coast of 

 Florida. (See map, Plate III.) They extend to and include the 

 Bahamas and other islands. These submarine plateaus have various 

 depths. An elevation of one hundred to three hundred feet would 

 greatly enlarge the Southeastern States, and raise the Bahama banks 

 into broad plains (in reality a continuation of the coastal plains of 

 the Southern States), separated from Cuba and Florida by only nar- 

 row channels. A lower broad plateau occurs in this same region, at 



