174 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
then the foot of a range of hills will be skirted, but the swamps continue 
on as far inland as Bujio. 
On the south sides of both Nava and Manzanilla Bays, for instance, 
the surfaces of these swamps stand about five feet above sea level, and 
the high swash of the waves is constantly undermining and destroying 
their substructure, here composed of a brownish dirty colored sand, which 
under the microscope shows about an equal part of fine grains of quartz 
(some of which are rounded, others quite jagged and angular) and small 
rounded grains of igneous rock, all of which are accompanied by numer- 
ous marine shells so recent in appearance that the nacreous tints are in 
many cases preserved. Inland, except along the line of the canal, it 
was impossible to ascertain the substructure of the swamp level. 
There is not sufficient free quartz in the structure of the adjacent 
land to have furnished the quartz sands in the swamp formation, and 
from its resemblance to the beach sands of to-day and arrangement in 
stratified bands, there is little doubt that the substructure of these 
swamps are not entirely alluvial accumulations, as might at first be sus- 
pected, but contain the beach sands of a littoral marine formation which 
has been elevated since Pleistooene time above the sea. "These sands 
not only border the immediate shore of the bays, but thoy indent the 
interior between and behind the first line of hills composed of older 
Tertiary sediments, around which they are deposited unconformably 
nearly as far inland as Gatun, 
The only light obtainable upon the thickness of the swamp deposit is 
along the line of the canal dredging where it crosses the swamp north of 
the Mindi Hills. Here from the bottom of the canal, 28 feet below sea 
level, the same sediments and shells similar to those seen at the sea 
margin have been dredged, and now thrown up in the terreplains which 
border the excavation along this portion of its course. The same char- 
acter of sedimentation was also seen up the deviation cut, two miles 
east of the mouth of the canal. In fact this formation has the same 
aspect a hundred miles west on the Costa Rican coast at Port Limon. 
I do not assert, however, that all the interior swamps are underlain by 
this marine formation, for some of them are at a higher level, and there 
is no way of ascertaining their geologic composition. The important 
fact is established, however, that bordering and indenting the coast of 
the Isthmus on the north side is a Pleistocene, or even Post-Pleistocene 
formation, which represents a former but comparatively recent inland 
extension of the sea, the sediments of which have been converted into 
land by a slight but comparatively recent elevation. The new land 
