REVIEWS 663 
The surface of Panama is described as consisting of irregularly 
rounded mountains and hills, 200 to 1500 feet in height. Their 
topographic expression is uniform whatever the rock of which they are 
composed. Lack of systematic arrangement in their distribution is 
said to be one of their striking features, though an east-west trend is 
locally observable. Their form and arrangement are ascribed pri- 
marily to erosion. 
“In common with the whole Central American region south of 
Yucatan, the Isthmus of Panama presents no such feature as a well- 
defined coastal plain like that bordering the eastern and southern 
margin of the United States. .... Such occasional levels as may be 
recognized on either coast are the products of the erosion of the 
greatly distorted sedimentaries and volcanic rocks. .... The Carib- 
bean coast is generally marked by jagged and abrupt bluffs where the 
sea beats directly against the hills. The indentations are slight and 
far apart. The same may be said of the Pacific side.” 
The drainage cf the Isthmus is defined as “ancient, mature and 
autogenous, consisting of deeply incised headwater ramifications 
drowned in their lower courses toward the sea.” Although the drain- 
age of the Isthmus is about equally divided between the two oceans, 
there is no well defined watershed separating the waters flowing into 
the Atlantic from those flowing into the Pacific. 
The Isthmian region is now undergoing rapid erosion, the result of 
the excessive rainfall (154 inches in 1894), and of the activity of the 
waves of the oceans. The topography of the sea bottom on either side 
points to a former greater expansion of land, and therefore to the fact 
that the narrow neck of land is, and has been disappearing under 
the influence of the agencies mentioned. This conclusion is further 
_ borne out by the outlying islands, which, by their structure and tela- 
tions, show themselves to be isolated remnants of the mainland. 
Two detailed geological sections are given, one across the Isthmus 
from Colon to Panama, and the other across Costa Rica. In the 
Isthmian section, seven structural units are recognized. These are (1) 
the fringing coral reefs; (2) the coastal swamps of both coasts — 
elevated plains of sedimentation; (3) the Monkey Hill and Panama 
benches— elevated base-leveled plains; (4) the folded and disturbed 
Tertiaries which owe their positions to the series of post-Tertiary (post- 
Oligocene) orogenic foldings along the Caribbean side of a more ancient 
nucleal region; (5) the numerous protrusions of basic igneous rocks, 
