GEOLOGY. 17 



Marine shells, all of existing species, are found in groups in the 

 post-pliocene sand, in a nearly horizontal stratum, and extend- 

 ing from its junction with the alluvium, to the depth of five 

 feet below it. They are generally immediately below the 

 fossil bones, but always in the sand, and in no instance found 

 in the clay alluvium. The depth at which the fossil bones 

 have been discovered varies with the different localities ; being 

 from five feet above high tides, to six or seven feet below. 

 The line of depth appears, however, to follow the present slope 

 of the land, as the highest deposit occurs farthest from the sea, 

 and the lowest nearest to it. The species of shells, and the 

 manner in which they are grouped together, are exactly such 

 as now occur on the adjacent coast ; and, as several of the 

 shells, such as the Artemis Concentrica and Tellina alter- 

 nata still retain the epidermis, it is obvious that they grew 

 on, or near the places where they are now found ; and that 

 the physical circumstances necessary to the existence of a 

 large portion of the present mollusca of the coast, have not 

 changed since a period long anterior to the extinction of the 

 gigantic mammalia. 



The fossil bones were found, generally in groups, and in seve- 

 ral instances the parts of the same skeleton were lying together. 

 The bones were generally entire, well preserved, and in no 

 instance abraded or incrusted with marine shells, except where 

 they had been washed out of the original bed into salt water 

 streams. These circumstances render it highly probable that 

 these animals either perished on the spots where their remains 

 are now found, or that their carcasses were quietly and im- 

 mediately floated to them, and that sinking to the sandy bot- 

 toms of the then shallow bays, lakes or streams, they were 

 gradually enveloped in the sedimentary deposits, which have 

 produced the older recent alluvium, or inland swamp forma- 

 tion. The inferences may also be drawn, that they existed at 

 a period posterior to the elevation from the sea, of a well 

 characterized sandy post-pliocene formation, and at the com- 

 mencement of the alluvium, of the age of the inland swamp : 

 and that their destruction must have been owing to gradual 

 changes in physical circumstances, and not to any sudden and 

 violent catastrophe. The occurrence in South America, in 



