288 c. w. HAYES — geology op Nicaragua canal route 



tween the volcanic ranges on the southwest and the Chontales hills on 

 the northeast. 



CLASSIFICATION OF TOPOGRAPHIC FEATURES 



Types in general. — Three distinct types of topography are encountered 

 in this portion of Central America, namely : 



Oldland areas with maturely developed degradational surfaces. 



Recent volcanic cones and plateaus with slightly modified construc- 

 tional surfaces. 



Recent floodplains and deltas with still forming aggradational surfaces. 



The oldland occupies much the larger portion of the region represented 

 on the accompanying map. It forms most of the San Juan valley, and 

 expands northward between the divergent lines of the Caribbean coast 

 and the Nicaragua-Managua lake basins. It also forms the narrower 

 portion of the land strip between the former lake and the Pacific. This 

 part of the region appears to have been above sealevel since the middle 

 of the Tertiary, and the form of its surface is due entirely to the action 

 of subaerial gradational forces. Although composed largely of volcanic 

 materials, the original constructional surfaces appear to be entirely 

 obliterated. 



The recent volcanic cones constitute two ranges — the Costa Rican 

 range, terminating in the volcano Orosi, and the parallel Nicaraguan 

 range, which extends from Madera northwest to the gulf of Fonseca. 

 The eruptions which gave rise to the Costa Rican range appear to have 

 broken out on a somewhat elevated land surface, perhaps similar in age 

 and topographic development to the present Chontales hills. The 

 northern series of eruptions, on the other hand, occurred on the sea 

 bottom along a line near the center of a bay which formerly indented 

 the Pacific coast. Most of the volcanoes constituting these two ranges 

 are extinct, or at least quiescent, but they are so recent that their con- 

 structional slopes are only slightly modified b}^ subsequent erosion. The 

 same is true of the gently sloping volcanic plateaus from which rise the 

 cones of the northern range. 



The floodplains and deltas form a comparatively small part of the 

 region, but their importance is out of proportion to their extent, partic- 

 ularly from the viewpoint of the canal engineer. By a recent depression 

 of the land all the streams entering the sea have been drowned and the 

 estuaries thus formed have been silted up. The deltaplains are the sea- 

 ward extension of the floodplains, and hence are the most recent topo- 

 graphic forms of the region, and the metliod of their formation may still 

 be observed in active progress, 



