SUMMARY AND CONCLUSION 347 



tion of the Toro rapids. It is evident that the dam formed of this 

 unconsolidated material is only very temporary, and that the backward 

 cutting of the river channel, unless artificially checked, will soon lower 

 this barrier and eventually affect the level of the lake. 



It is difficult to determine exactly the position of the old divide. It 

 undoubtedly crossed the valley of the present San Juan below the mouth 

 of the Poco Sol. That stream has evidently inherited the lower portion 

 of its course from a tributary to the stream flowing northwest. The 

 Santa Cruz also probably belonged to the western drainage. The gen- 

 eral course of the Bartola, on the other hand, indicates that it belonged 

 to the eastern system ; hence the divide Avas probably between the Bar- 

 tola and the Santa Cruz. It may have been at the present Castillo 

 rapids, although it is probable that the rapids would show some reces- 

 sion due to erosion since the lake was formed. This, however, might be 

 comparatively little by reason of the character of the rocks and the fact 

 that the river at this point carries comparatively little coarse sediment, 

 and hence is relatively inefficient in corrading its channel. 



Summary and Conclusion 



1. The region discussed embraces the belt of country extending from 

 the Caribbean sea to the Pacific in northern Costa Rica and southern 

 Nicaragua adjacent to the route of the proposed Nicaragua canal. 



2. Its most important physiographic feature is the broad depression 

 which extends diagonally across the isthmus between the recent volcanic 

 ranges on the southwest and the Chontales hills on the northeast. The 

 topography of this depression is chiefly that of an oldland, generall}^ re- 

 duced to the condition of a peneplain by streams flowing in opposite 

 directions from a former divide near the axis of the isthmus. 



3. The rainfall on the Caribbean side of the isthmus is very abundant 

 and distributed uniformly throughout the year. On the Pacific side it 

 is less abundant and confined to half the year. This climatic difference 

 produces striking differences in vegetation, rock decay, rate of erosion 

 and resulting topographic forms. 



4. The rocks of the region are largely volcanic products, with two sedi- 

 mentary formations of Tertiary (Oligocene) age, and no rocks occur which 

 are certainly older than the Tertiary. The igneous rocks are in part 

 contemporaneous with the Tertiary sedimentary formations and in part 

 recent. 



5. On the east side high temperature with abundant moisture and 

 consequent rank and rapidly decaying vegetation afford exceptionally 

 favorable conditions for rock decay, which has extended to great depths 



