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



Page 



Tertiary igneous rocks 315 



Location and general character 315 



Massive igneous rocks 316 



Fragmental igneous rocks 317 



Recent alluvial formations 319 



Recent volcanic rocks 320 



Rock decay 322 



Importance of the subject 322 



Conditions favoring rock decay 322 



General discussion of the subject 322 



Effect of chemical composition 323 



Effect of original structure 324 



Effect of secondary structures 324 



Rock decay in the eastern division 325 



Products of rock decay 326 



Classes in general 326 



Red clay 326 



Blue clay 328 



Soft rock (saprolite) 328 



Rock decay in the western division 329 



Recent geological history 331 



Relationship between topography and geology 331 



Conditions anterior to Tertiary time 331 



Early Tertiary deposition and volcanic activity 331 



Middle Tertiary uplift and erosion 333 



Post-Tertiary elevation and gorge cutting 335 



Recent depression and alluviation 339 



Formation of lake Nicaragua 340 



Subsequent modification of the lake 344 



Volcanic eruptions 344 



Wave cutting 344 



Alluviation 345 



Summary and conclusion 347 



Introduction 



The region discussed in the following paper embraces northern Costa 

 Rica and southern Nicaragua. Its special interest lies in the fact that 

 it contains the lowest gap in the Continental divide between the straits 

 of Magellan and the Arctic ocean, and probabl}^ also contains the most 

 feasible route for an interoceanic ship canal. 



In connection with the investigations of the canal route b_y the United 

 States Nicaragua Canal Commission, the writer spent ten months of 1898 

 in field-work in tlie canal region, the greater part of which was in the 

 direction of drilling operations. In this way much detailed information 



