294 C. W. HAYES — GEOLOGY OP NICARAGUA CANAL ROUTE 



they can be inferred, are represented on the accomjoanying sketch map 

 (plate 30). It will be noted that lake Nicaragua did not then exist. Its 

 present basin was occupied in part by a bay indenting the coastline and 

 in part by the basins of rivers tributary to this bay. The region occu- 

 pied by the volcanic peaks of the Nicaraguan range and the volcanic 

 plateau west of the lake was then occupied by the sea. A cape project- 

 ing northward between the sea and the bay was composed of low hills, 

 now forming the Continental divide southwest of the lake. 



In still other respects the drainage of the region during the formation 

 of this peneplain differed from the present. The San Juan river receives 

 only small tributaries from the north, while it receives both small and 

 large from the south. The large tributaries include the Frio, Poco Sol, 

 San Carlos, and Sarapiqui. These all head on the slopes of the Costa 

 Rican volcanic range, which forms the southern margin of the Nicaraguan 

 depression. The upper portions of these streams are normal to the 

 mountain range, the axes of their valleys being at right angles to the axis 

 of the range, and also to the geiieral course of the San Juan. Midway 

 of their courses, however, there is an abrupt change in direction. The 

 Frio and Poco Sol bend westward, while the San Carlos and Sarapiqui 

 bend eastward, the axes of the lower valle3^s in every case making a 

 rather acute angle with the course of the San Juan. It seems probable 

 that when the peneplain was being developed in this region the two 

 rivers whose basins now form that of the San Juan occupied the axes of 

 those basins, receiving tributaries of equal length from either side. The 

 volcanic eruptions to the south, however, obliterated the former drain- 

 age of that region, and the consequent streams developed on the flanks 

 of the newly formed mountains were turned northeast, discharging into 

 the heads of the preexisting small tributaries. It thus ajDpears that the 

 four above named southern tributaries of the San Juan have composite 

 courses. Their upjoer courses, normal to the trunk stream, are conse- 

 quent on the constructional slope of the recent volcanic range ; their 

 Tower courses, making acute angles with the trunk stream, are inherited 

 from the normally developed, small tributaries of two streams flowing 

 respectively southeast and northwest. 



The rapidity with which the streams heading upon the Continental 

 divide reduced their valle.ys to baselevel depended chieflv upon the 

 character of the rocks which they encountered, while the rate at wliich 

 the divide was lowered b}^ the action of opposing streams depended on 

 the character of the rocks and the distance of the divide from the coast 

 or the width of the isthmus. The region to the northward is probably 

 occupied by the older and more resistant rocks, including gneisses, 

 schists, and quartzites. Of that to the south very little is known, since 



