100 
sea level since the middle of the Tertiary, 
and its form is due entirely to the action of 
subaerial gradational forces. Although 
composed largely of volcanic rocks, the 
_ original constructional surfaces appear to 
have been entirely obliterated. 
When examined in detail the old-land 
surface is found to have considerable diver- 
sity in its relief, and its topographic forms 
naturally fall into three classes. These are 
(1) fairly well developed and subsequently 
dissected peneplains whose remnants rise 
gradually from either coast toward the axis 
of the isthmus, which until recent geologic 
times was occupied by the continental 
divide; (2) residual hills or monadnocks 
which rise distinctly above the peneplain 
surface, being most numerous toward the 
axis of the isthmus along the former con- 
tinental divide, and (3) many valleys which 
intersect the peneplain surface, having been 
cut during a period of high level and sub- 
sequently somewhat depressed. 
The peneplain was most extensively de- 
veloped near the east coast, but it has here 
suffered most from subsequent erosion, and 
only a few remnants occur along the lower 
portion of theSan Juan Valley. Higher up 
the valley, at a distance of 40 or 50 miles 
from the Caribbean coast, the peneplain was 
well developed, and, although deeply dis- 
sected, the even summits of the hills, about 
150 feet above sea level, and considerable 
areas of level country back from the river, 
give evidence of its former extent. To the 
west of the former divide a corresponding 
peneplain is found sloping gently westward. 
It forms a plain of variable width about the 
lower portion of Lake Nicaragua and doubt- 
less extends beneath the waters of the lake. 
As indicated above, the residual hills are 
most numerous near the position formerly 
occupied by the continental divide, being 
separated by low colls, in which opposing 
streams headed, slightly above the level of 
the peneplain. The San Juan Valley, 
SCIENCE. 
[N. S. Von. X. No. 239. 
where it crosses this monadnock belt, is 
very narrow and bordered by high hills 
with serrate outlines totally unlike the low 
even-topped hills on either side. The 
monadnocks increase in height and num- 
bers toward the north, forming the Chon- 
tales hills, which merge with the mountains 
of northern Nicaragua. The latter reach 
altitudes of 5,000 to 7,000 feet. 
The valleys which intersect the surface 
of the peneplain were cut when the land 
stood at least 200 feet higher than now. 
They have a much steeper gradient than 
the present streams, showing that the high- 
level period was not long enough for the 
streams to reduce their channels to base- 
level. A subsequent depression of the land 
drowned the lower portions of the valleys. 
On the Pacific side, where the coast is bold 
and rugged, these drowned valleys are 
comparatively narrow and filled with allu- 
vium nearly or quite out to the rocky head- 
lands, but the alluvium never extends sea- 
ward beyond the latter to form a delta. On 
the Caribbean side the rivers are longer and 
deliver more sediment than the waves and 
littoral currents can dispose of, and hence 
have not only filled the estuaries formed in 
the drowned valleys, but have built out a 
series of deltas which coalesce and form a 
coastal plain. The sediment brought down 
to the sea by streams north of the San Juan 
is small compared with that brought down 
by those to the south. The more rapidly 
growing southern deltas would, therefore, be 
extended seaward except for a strong north- 
ward sand current set up by the oblique 
direction at which the waves strike the 
shore. This sand current tends to dis- 
tribute the sediment evenly along the coast 
and preserve gently curving coast lines. 
Sediment, however, is delivered by the San 
Juan slightly faster than it can be dis- 
tributed. Hence it tends to build out a 
delta, but this is deflected to the northward 
and forms a curved sandspit which for a 
