142 CHARLES DARWIN 



water, gradually encroached on, and at last converted into 

 the bed of a muddy estuary, into which floating carcasses 

 were swept. At Punta Gorda, in Banda Oriental, I found 

 an alternation of the Pampsean estuary deposit, with a lime- 

 stone containing some of the same extinct sea-shells; and 

 this shows either a change in the former currents, or more 

 probably an oscillation of level in the bottom of the ancient 

 estuary. Until lately, my reasons for considering the Pam- 

 paean formation to be an estuary deposit were, its general 

 appearance, its position at the mouth of the existing great 

 river the Plata, and the presence of so many bones of ter- 

 restrial quadrupeds: but now Professor Ehrenberg has had 

 the kindness to examine for me a little of the red earth, 

 taken from low down in the deposit, close to the skeletons 

 of the mastodon, and he finds in it many infusoria, partly 

 salt-water and partly fresh-water forms, with the latter 

 rather preponderating; and therefore, as he remarks, the 

 water must have been brackish. M. A. d'Orbigny found on 

 the banks of the Parana, at the height of a hundred feet, 

 great beds of an estuary shell, now living a hundred miles 

 lower down nearer the sea; and I found similar shells at a 

 less height on the banks of the Uruguay; this shows that 

 just before the Pampas was slowly elevated into dry land, 

 the water covering it was brackish. Below Buenos Ayres 

 there are upraised beds of sea-shells of existing species, 

 which also proves that the period of elevation of the Pam- 

 pas was within the recent period. 



In the Pampaean deposit at the Bajada I found the osse- 

 ous armour of a gigantic armadillo-like animal, the inside 

 of which, when the earth was removed, was like a great 

 cauldron; I found also teeth of the Toxodon and Mastodon, 

 and one tooth of a Horse, in the same stained and decayed 

 state. This latter tooth greatly interested me,* and I took 

 scrupulous care in ascertaining that it had been embedded 

 contemporaneously with the other remains; for I was not 

 then aware that amongst the fossils from Bahia Blanca 

 there was a horse's tooth hidden in the matrix: nor was it 

 then known with certainty that the remains of horses are 



* I need hardly state here that there is good evidence against any horse 

 living in America at the time of Columbus. 



