680 THE AMERICAN NATURALIST. 
“In early Tertiary (Oligocene) time there was probably free com- 
munication 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. 
“In middle Tertiary time the region was elevated and subjected to 
long-continued subaérial 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 imus 
since the middle Tertiary uplift. 
“In post-Tertiary time the region was again elevated, and the 
previously developed peneplain deeply trenched. 
“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. 
“ Recent volcanic eruptions have formed a barrier across the out- 
let 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 continental 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.” 
Accompanying the paper is a map of the region adjacent to the 
canal route, showing the positions of active and extinct volcanoes, 
present and former continental divide, former Pacific coast line, 
proposed canal route, etc.; and a sheet of geological sections. 
ISRAEL C. RUSSELL, 
