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 Pleistocene time above the sea. These sands 

 not only border the immediate shore of the bays, but they 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 di'edged, 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. Iu fact this formation has the same 

 aspect a hundred miles west on the Costa Bican 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 



