JULY 28, 1899. ] 
continued until late Tertiary time, when the 
region was elevated, a land barrier cutting 
off connection between the two oceans, which 
has never been restored. After a long 
period of quiesence, during which extensive 
peneplains were developed on both sides of 
the continental divide then occupying the 
axis of the isthmus, the region suffered an- 
other elevation and the peneplains were 
deeply trenched by river valleys. This 
period of gorge cutting was followed by a 
subsidence equal to about half the previous 
uplift. The river valleys were drowned, 
and the estuaries thus formed have since 
been in part or entirely silted up. 
The renewal of volcanic activity in late 
Tertiary or post-Tertiary time gave rise to 
the two mountain ranges above described. 
The position of the northern series of vents 
with reference to the coast line was such 
that when their ejected material had 
reached the surface of the sea it formed a 
barrier. across the bay which then indented 
the Pacific coast. This barrier was built 
gradually higher by successive eruptions, 
and since in the area behind it precipita- 
tion was greater than evaporation the 
waters rose above sea level and doubtless 
escaped westward over the barrier during 
periods of quiesence in the volcanic ac- 
tivity. As the surface of the barrier 
was raised by the addition of volcanic 
ejecta, the surface of the impounded waters 
was raised to a height probably somewhat 
above the present elevation of Lake Nica- 
ragua. The lake thus formed occupied not 
only the position of the former bay, but 
flooded the basins of the tributary streams 
and was considerably larger than the pres- 
ent Lakes Managua and Nicaragua com- 
bined. Its surface finally reached a low 
point in the continental divide where a 
west-flowing stream headed against one 
which occupied the present position of the 
San Juan. When this point was reached 
the intermittent escape of the impounded 
SCIENCE. 103 
waters across the volcanic dam to the west- 
ward was changed for a permanent outlet 
to the eastward. 
The gap when first discovered and over- 
topped by the rising waters was doubtless 
in deeply weathered rock and residual clay. 
It must, therefore, have been cut down very 
rapidly until the underlying hard rock was 
reached, when the permanent level of the 
lake was established which it has retained 
practically unchanged to the present time. 
It is quite possible that the gaps through 
the continental divide to the east and 
through the divide between the lake and 
ocean to the west were so near the same 
level that the impounded waters had for a 
short time an outlet both to the Atlantic 
and to the Pacific. The upper Rio Grande 
is flowing in a partly silted-up rock gorge 
much too large for the present stream, and 
it appears probable that this gorge was cut 
by the outflow from the lake before it was 
entirely and permanently diverted to the 
eastward outlet. 
Certain features which have a specially 
direct bearing upon the canal problem 
should receive a further word of descrip- 
tion. One of these is the gap followed by 
the canal route between the lake and the 
Pacific, the lowest gap in the continental 
divide between the Arctic Ocean and the 
Straits of Magellan. 
The lower portion of the lake is bordered 
by the peneplain above described which, in 
the vicinity of Rivas, is very perfectly base- 
leveled. The plain rises gradually west- 
ward from the lake shore to the range of 
hills which forms the divide. These hills 
are from 500 to 1,200 feet high and extend 
northward to a point opposite the Island 
Zapetero where they meet the Jinotepe 
plateau and their residual old-land forms 
give place to the even constructional slopes 
of the latter. A single break occurs in this 
continuous line of hills, forming the gap be- 
tween the waters of the Rio Lajas and the Rio 
