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



and yields red clay as the final product. On the west side alternate wet 

 and dry seasons afford less favoral^le conditions for rock decay, and the 

 final product is blue clay. 



6. In early Tertiary (Oligocene) time there was i)robably free commu- 

 nication across this portion of the isthmus between the Atlantic and the 

 Pacific. A great mass of sediments was deposited in a shallow sea and 

 many volcanoes were in active eruption. 



7. In middle Tertiary time the region was elevated and subjected to 

 long continued subaerial degradation, and the narrower portion of the 

 isthmus was reduced to a peneplain, with monadnocks at the divide near 

 the axis. There is no evidence that open communication has existed 

 between the two oceans across this portion of the isthmus since the middle 

 Tertiary uplift. 



8. In post-Tertiary time the region was again elevated and the pre- 

 viously developed peneplain deeply trenched. 



9. A recent slight subsidence has drowned the lower courses of the 

 river valleys, and the estuaries thus formed have subsequently been 

 filled with alluvial deposits. 



10. Recent -volcanic eruptions have formed a barrier across the outlet 

 of a bay which formerly indented the Pacific coast. The waters rose 

 behind this barrier until they reached the level of a low gap in the Con- 

 tinental divide, when they discharged to the eastward, and the divide 

 was shifted to the newly formed land near the Pacific coast. Lakes 

 Managua and Nicaragua thus occupy the bed of the former bay and the 

 basins-of rivers which were tributary to it. 



