170 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
experience in observing the phenomena I have never seen such an ad- 
vanced example of a decaying region. 
Topography of the Submarine Bench. — In considering the Isthmian 
topography it is necessary to extend our profile seaward in both direc- 
tions in order to understand the true contour of the cross section, for on 
both sides there exists a shallow submarine bench, which must have been 
a continuation of the land in these directions. "The submarine bench of 
the Caribbean is a gradually sloping plain for many miles northward of 
Colon until the depth of 500 fathoms is reached, presenting a flattened 
gradient. Then the profile makes a great plunge from 600 to 1,900 
fathoms, presenting a submarine escarpment which, although of great 
height, is not so abrupt as that of the Pacific. 
On the Pacific side the waters of Panama Bay are so shallow that 
their average deepening does not exceed one fathom per mile, until 
the 100 fathom line is reached, nearly 100 miles south of Panama city. 
This line will almost connect Cocalita Point and Cape Mala, the two 
points which mark the entrance to the Gulf. Here, however, almost 
coincident with the abrupt Pacific coasts, there is a gigantic submarine 
escarpment plunging off into the Pacific to the depth of 1,700 fathoms 
or more. These features we have added to the profile of the land on 
Plate IV. Figure 2. 
The submarine topography of Panama Bay is also of deepest interest 
in interpreting the history of the former magnitude and decay of the 
land. Not only do the numerous islands stand as monuments to its de- 
cay, but the floor of the bay itself reveals a remarkable surface. i have 
carefully plotted upon the map, as shown in Plate IV., the submarine 
contours of Panama Bay. The results clearly show the presence of 
a topography remarkably similar to that of the land, including several 
deep arterial channels which may have been submerged river valleys. 
This submarine topography may represent the continuation of the larger 
rivers many miles out into the region now covered by the Pacific waters, 
hat the ramifying systems of drainage now visible 
and strongly suggests t 
eadwater portion of what were once far more 
are but remnants of the h 
extensive streams. 
There are two great factors in the destruction of the land of the Isth- 
mian region: Ist, the excessive rainfall; and, 2d, the marine erosion of 
the waters or the Caribbean and Pacific. 
The rainfall of the Isthmian region, with the accompanying erosion, 
is excessive, exceeding any other upon the western hemisphere. For the 
year 1894, the one preceding my visit, according to the measurements 
