336 C. VV. HAYES (JEOLOGY OF NICAEACJUA CANAL ROUTE 



as far up as the San Francisco have lowered their channels below their 

 old baselevel — if not entirely to their headwaters, at least Avell back 

 toward them. Beyond the San Francisco the upper portions of the 

 tributaries are found still flowin<2; at the level of their old valleys, which 

 they have not as yet had time to completely dissect. Excellent exam- 

 ples of this immature drainage are seen in the basin of the Machado, 

 and with increasing frequency from that point westward to the Toro 

 rapids. Thus the Machuca and Bartola are rapid streams, still actively 

 corrading their channels almost down to their junction with the San 

 Juan, 



The stream which occupied the upper portion of the San Juan valley, 

 as indicated above, headed on the Continental divide in -the vicinity 

 of Castillo, and receiving as tributaries the Rio Frio and other streams 

 now emptying into the lower end of the lake, flowed northwest to the 

 head of a bay in the vicinity of the island of Madera. This stream, like 

 the other, was stimulated by the uplift and rapidly cut its channel back- 

 ward, dissecting its old valleys well up toward the Continental divide. 

 This old channel, now drowned by the waters of lake Nicaragua, has 

 been traced more or less continuously from the vicinity of Madera south- 

 east with gradually decreasing depth to the vicinity of the Balsillas 

 islands. It may very likely have extended beyond this point, and its 

 upper portion has been subsequently filled by the sediment carried into 

 the southern end of the lake chiefly by the Rio Frio. 



The cape which extended northwest between the waters of the Pacific 

 ocean and the bay of Nicaragua appears to have suffered some differen- 

 tial uplift, its southern portion being elevated more than its northern 

 portion. Not enough study has yet been given to the whole of this re- 

 gion, however, to determine with any degree of certaint}^ the details of 

 its recent history. Nevertheless it is known that the rivers to the south 

 of the Rio Grande have cut their channels much deeper than those to 

 the north, and that some of the latter appear to have been affected but 

 little either by this uplift or by the subsequent depression. Onl_y the 

 valley of the Rio Grande has been carefully studied, and it is certain 

 that the uplift there was at least 200 feet. 



The active wave cutting along the Pacific coast during this and the 

 ])receding period shortened the distance from the coast to the subordi- 

 nate divide on the highland forming the cape, thus rendering the length 

 of the streams flowing in opposite directions from this divide verj^ un- 

 equal. Those flowing to the Pacific therefore had a very steep gradient, 

 while those flowing east to the Nicaraguan depression had a compara- 

 tively flat slope; hence the corrasion of their channels was proportion- 

 ately greater by the steams flowing directly to the Pacific than by those 



