1847.] JVotes on Natural History. 257 



ance composed of marly clay mingled with sand. The upper 

 portion is much broken up, but no fossils are visible at this place. 

 The ninth and upper stratum varies exceedingly in thickness, 

 being at this point about ten feet, while at other locations it fre- 

 quently exceeds one hundred, and is chiefly made up of rounded 

 pebbles, consisting of fragments of granite, trappean rocks, por- 

 phyry, quartz, jasper, pumice, and agates in abundance. This 

 great covering of gravel, has in many places been entirely re- 

 moved by the action of flowing waters over the surface of the 

 plain at some far distant era, so that the stratum of sand and clay 

 from beneath, has been frequently exposed to view, and in some 

 instances to no inconsiderable extent. 



The numerous fossil organic remains found in the above section, 

 are characteristic of the Tertiary period, and with the exception 

 of the giant mammalia, in all probability to its most ancient strata. 

 In the more recent deposites of these plains, and evidently 

 formed by the disintegration of its strata, are at present to be 

 found the remains of some of the most remarkable gigantic an- 

 imals that have ever inhabited the globe, and which at some far 

 distant period of time held undisputed sway, and throve luxuriant- 

 ly upon the vegetation that once had an existence in this region 

 of country, now so bare and desolate to the sight. They are to 

 be discovered in the greatest profusion, associated w^ith numerous 

 recent species of shells, which may yet be ibund both on the land 

 and in the sea in this vicinity, furnishing to the mind convincing 

 evidence, that they lived and passed away at a comparatively 

 recent era of the earth's history, without leaving any other indi- 

 cations of their existence. In some few instances, even the gen- 

 era to which these animals belonged, are no longer to be met 

 with on this great continent of the west. Those most commonly 

 met with, belong to the following genera, and many others have 

 been found and described by naturalists. They are nearly all 

 about the size of the elephant, and most of them are represented 

 by diminutive species still living about these plains: mastadon, 

 megatherium, megalonyx, mylodon, toxodon, scelidotherium and 

 equus. 



Skirting the shores of the sea and extending beneath its waves 

 to the distance of about one hundred miles to the east, is to be 

 found a continous plain, and from what little is known of its na- 

 ture, it appears to exhibit in a peculiar manner indications of a mnch 

 lower and similarly constituted terrace, or step. It extends along 

 the South American coast for an immense distance, and is famil- 

 iarly known as the Brazilian and Patagonian banks. At its east- 

 ern termination it falls abrubtly into the profound depths of the 

 ocean, and for nearly its whole extent supports an average mass 

 of about sixty fathoms of water. 



