102 
important factors in the canal problem. 
Through the greater part of the year the 
trade winds prevail with fairly constant 
direction and force. They are deflected 
slightly to the north by the high volcanic 
range of Costa Rica and to thesouth by the 
mountains of northern Nicaragua. The 
low gap across the isthmus constituting the 
Nicaraguan depression thus receives more 
wind than would be due to the normal 
trades, and it is probably this congestion of 
the air currents that causes the exceptional 
precipitation of this region. Within the 
zone of maximum precipitation which em- 
braces the coastal plain and adjacent hills, 
forming a belt from 50 to 75 miles broad, 
the annual rainfall reaches nearly 300 
inches. Beyond this belt, with increasing 
distance from the Caribbean coast, it de- 
creases very rapidly, and in the western 
portion of the region the average annual 
rainfall is less than a third and in some 
seasons less than a tenth of that on the 
eastern coast. More important, however, 
than the absolute amount of rain is its dis- 
tribution throughout the year. In the 
eastern division the rain is distributed with 
tolerable uniformity through the year. In 
the western division, on the other hand, 
there is a distinct dry season of five or six 
months. These climatic differences give 
rise directly to very striking differences in 
vegetation and, either directly or indirectly, 
to differences in the appearance and struc- 
ture of the soils, in the topographic forms 
of the land surface and in the effectiveness 
of various physiographic processes. 
The eastern division is covered by a dense 
tropical forest wherever the land is suffi- 
ciently firm to support large trees. The 
falling rain is intercepted by the canopy of 
foliage and filters gradually down to the 
surface, where the smaller vegetation affords 
a further protection, so that the soil never 
receives the direct impact of the rain drops. 
The abundant forest litter decays rapidly, 
SCLENCE, 
[N. S. Von. X. No. 239. 
furnishing a constant supply of the complex 
organic acids which are chiefly instrumental 
in promoting rock decay, and the latter pro- 
cess is extremely active. Solid rock is 
rarely found, except in residual bowlders, 
at depths less than 40 to 100 feet from the 
surface. The prevailing soil is a very tena- 
cious, residual, red clay which never be- 
comes dry enough to be intersected by 
shrinkage cracks and which, although to 
some extent loosened by roots and insects, 
resists erosion to a remarkable degree. 
After a careful study of the region it was 
concluded that the absence of frost and the 
presence of the tropical forest more than 
counterbalance the enormous rainfall and 
that surface degradation is, on the whole, 
slower than in most temperate regions. 
The western division, particularly that 
portion lying between the lakes and the 
Pacific, is characterized by open savannahs 
and the thin foliaged, thorny forests of a 
semi-arid region. The forest litter is mostly 
destroyed by fires during the dry season, so 
that rock decay is hindered and the soil is 
wholly unprotected from the torrential rain- 
fall which inaugurates the wetseason. The 
soil, which is never red, but generally dark 
blue, is alternately intersected by shrinkage 
cracks and saturated with water, a process 
which serves to loosen it almost as effectu- 
ally as frost. It results that the streams, 
which are alternately rivulets and torrents, 
bear great quantities of detritus, and the 
surface degradation is comparatively rapid. 
No sedimentary or other records have 
been found in this portion of Central Amer- 
ica which carry its history back to an 
earlier period than the Tertiary. During 
the Oligocene there was probably free com- 
munication between the waters of the At- 
lantic and Pacific, the region of the Nicara- 
guan depression being occupied by a shal- 
low sea in the vicinity of which were many 
active volcanoes. The extrusion of volcanic 
materials and the deposition of sediments 
