340 C. W. HAYES — GEOLOGY OF NICARAGUA CANAL ROUTE 



accomplished during the depression of the land surface. It will be re- 

 called that the Rivas plain is a plain of degradation, formed by the action 

 of streams flowing near baselevel. A plain formed in this way must 

 necessarily be nearly horizontal, but the present Rivas })lain has a sloi)e 

 to thejiortheast of about 8 feet to the mile. This is manifestly greater 

 than the gradient of streams forming a baseleveled plain. It is consid- 

 erably greater than the gradient of the present streams which cross it. 

 The latter, emerging from rather deep, narrow gorges in the residual hills 

 to the southwest, cut narrow channels in the inner portion of the Rivas 

 plain. These channels in some cases have a depth of 60 feet or more. 

 The}'- gradually decrease in depth toward the outer margin of the i)lain, 

 the unequal slopes of the stream bed and the peneplain surface bringing 

 them together at the lake margin. 



Accompan3nng the depression of the land which inaugurated this 

 period was a renewal of the volcanic activity of the region. It is possi- 

 ble that the volcanism and the depression may be intimately related as 

 cause and effect, or may be both the effects of a common cause. How- 

 ever this may be with regard to the depression of the region as a whole, 

 it is more than probable that the observed deformation of the surface is 

 due directly to the volcanic activity. This activity was manifested along 

 two lines of vents forming the lines of volcanic craters whose topography 

 has already been described. The southern series of vents forming the 

 Costa Rican volcanic range broke out within a land area and possibly 

 on a somewhat elevated plateau. These volcanoes have obliterated the 

 preexistent topography and built up a massive mountain range. The 

 northern series of vents forms the Nicaraguan volcanic range. Between 

 the nearest peaks of the two ranges — Orosi to the south and Madera to the 

 north — there is a gap of about 30 miles. 



However closely the two ranges may be associated in the causes which 

 led to the extension of their lavas and in the character of the lavas tiiem- 

 selves, they are entirely distinct at the surface and are separated bj'' sedi- 

 mentary and igneous rocks belonging to an earlier geological period. As 

 seen from the map (plate 30), on which the former position of the Pacific 

 coastline is shown, the volcanic vents which formed the Nicaraguan 

 range broke out on the sea bottom and extended nearly parallel to the 

 west coast. The northern vents of the group were the more active, and 

 have given rise to a somewhat continuous mountain chain and also to 

 the extensive Jinotepe plateau. 



FORMATION OF LAKE NICARAGUA 



The position of these volcanic vents with reference to the coastline was 

 such that when their ejected material had reached the surface of the sea 



