CENTRAL AMERICAN GEOLOGICAL WATER WAYS. 585 



a thousand feet; and indeed an elevation of seven hundred and 

 seventj-six feet has been enough to withdraw the waters from the last 

 canal across the continental barrier. But of this again. 



The same story of the rise of the land and the separation of the 

 seas is repeated in Central America — in Honduras, in Nicaragua, and 

 in Colombia, whether at Pana- 

 ma or not, as appears from the 

 observations of Mr. J. Crawford, 

 Dr. W. M. Gabb, Dr. G. A. 

 Maack, and especially of Mr. F. 

 C. ISTicholas, in the Atrato Val- 

 ley. But the phenomena accom- 

 panying the great changes of 

 level have not been so fully 

 studied farther south as in 

 Mexico. 



The Manner in which the 

 Elevation of the Plateau oc- 

 cuERED. — There are various 

 great terrestrial movements 

 known to geologists. Exten- 

 sive continental areas are gently 

 rising and others are sinking. 

 These oscillations, so far as time 

 is concerned, appear slow — as, 

 for instance, the sinking of the 

 New Jersey coast at the supposed 

 rate of a foot in half a century, or 

 of the New Orleans district, of 

 a foot in twenty years; while 

 the rising of the northern part of 

 the continent, as between the Adi- 

 rondack Mountains of New York 

 and the St. Lawrence River, is at the rate of a foot in twenty or 

 twenty-five years. Within a generation the changes of level in the 

 physical geography of a special district are no more apparent to the 

 ordinary inhabitant than the shallowing or deepening of small chan- 

 nels which interfere with or favor the passage of canoes or small 

 boats, or the gentle backing of waters which make swampy grounds 

 or the reverse. Even with their effects, accumulated for centuries, 

 the changes do not often affect the course of the drainage or obstruct 

 it. By this gentle continental movement the mountains are not 

 formed, nor are ridges squeezed up into existence, although they 

 often give rise to high land. Since the beginning of the Glacial 



Fig. 6. — Falls below Atoyac, passing 

 through a short, deep caiion, descending 

 from the terrace plain faintly shown in 

 the high background of the i)icture. 



