GEOLOGY AND MINERALOGY. 
The geological structure of the Isthmus is less clearly marked 
than that of the adjoining more elevafed districts of Mexico. A 
country having a horizontality of 135 miles from sea to sea, with 
an average elevation of not more than 600 feet, cannot present 
over its surface many exposed situations where the dip, strike, 
and nature of the rocky basis could be ascertained ; much of the 
gentle slope of the northern plains being covered over with clay, 
sand, and gravel, and so densely wooded, that no appearance of 
the rock formation is discernible. The same may be said of 
the plains on the southern shore. On the middle more elevated 
regions the conformation is more evident ; and what is de- 
ficient upon the sides of the hills, owing to the superficial de- 
posits of gravel and sand, has been in great part exposed by 
the sections made naturally, on the large scale in the mount- 
ain passes. 
The tertiary clays, gravels, and beds of detritus which cover 
up so much of the Isthmus along the line of survey, extend on 
the north side almost to the summit-level, and the base of the 
hills which lie east and west of it. These deposits being found 
pretty uniformly spread, even to the depth of thirty feet in some 
places, as at a point north of the summit-level, and between it 
and the river Almoloya, are evidences of the slow and tranquil 
elevation of this portion of the Isthmus above the sea, and of its 
comparatively quiescent condition since a very distant epoch. 
Granite and granitiform rocks do not occupy much extent of 
surface upon the Isthmus, and perhaps were not the chief up- 
heaving agents which elevated this district. Porphyritic, dioritic f 
