The Geological Society of America. — Hovey. 95 
Nicaragua canal and the largest lakes of the western hemisphere south 
of the glaciated region of North America. 
The region is characterized by two types of topography, viz: the 
recent volcanic mountains and plateaus in which the original con- 
structional forms are more or less perfectly preserved and the areas 
of Tertiary igneous and sedimentary rocks in which the forms are 
due to long continued subaerial erosion. A noteworthy feature is the 
absence of any continuous mountain range or chain of dominant 
peaks in this portion of the isthmus. 
A shallow depression occupies the western portion of the region, 
its larger axis being nearly parallel with the Pacific coast. This con- 
tains the lakes Nicaragua and Managua. The former is no miles 
in length with an area of 3,000 square miles, and a mean altitude 
of 106 feet. Its greatest depth is 200 feet. 
The climate of the region is tropical and insular, the annual range 
of temperature being small. The rainfall is greatest on the east coast, 
being nearly 300 inches at Greytown, and decreases somewhat uni- 
formly westward, being less than 80 inches on the west coast. Con- 
nected with the decrease in rainfall there is a striking change in the 
character of the vegetation, the dense tropical jungle of the east coast 
giving place to open forests and savannahs in the west. 
No rocks older than the Tertiary are found along the line of the 
canal. They consist of eruptive and sedimentary formations, the form- 
er including basalt, andesyte and dacyte and the latter calcareous 
sandstones and shales. In addition to these Tertiary rocks there are 
extensive recent alluvial deposits and the tufifs and lavas of the mod- 
ern volcanoes. 
The conditions throughout this region, but particularly in it'- 
eastern portion are favorable for rock decay and the regolith is un- 
usually extensive. In the eastern section it consists of red clay at the 
surface with a depth of 10 to 30 feet, then blue clay with a somewhat 
greater depth passing downward into thoroughly weathered rock 
or saprolite in which the original structure is more or less perfectly 
preserved. The depth to hard rock varies from 50 to 150 feet. In 
the western section the regolith is much thinner and the red clay is 
almost entirely wanting the residual products being blue or gray. 
The late geological history of the region so far as it can be made 
out. is briefly as follows: In early Tertiary time this portion of the 
isthmus may have been wholly submerged. At any rate marine sedi- 
ments were deposited throughout a considerable part of its extent 
and this was accompanied by intense volcanic activity. In middle 
Tertiary time there Vv'as an uplift and long continued erosion, tlie 
constructional volcanic topography being obliterated and the region, 
at least toward the south, being reduced to one of low relief. The 
present basin of lake Nicaragua was tlien occupied in part by a gulf 
connected with the Pacific to the northwest, and in part by the val- 
leys of tributary streams. The continental divide then occupied the 
hilly or mountainous region cast of the lake, crossing the p:-fscnt 
