352 Pampean Formation. paet ii. 



entertain the smallest doubt, is that the Pampean for- 

 mation was slowly accumulated at the mouth of the 

 former estuary of the Plata and in the sea adjoining it. 

 I have come to this conclusion from the reasons assigned 

 against the two foregoing theories, and from simple 

 geographical considerations. From the numerous shells 

 of the Azara labiata lying loose on the surface of the 

 plains, and near Buenos Ayres embedded in the tosca- 

 rock, we know that this formation not only was formerly 

 covered by, but that the uppermost parts were deposited 

 in, the brackish water of the ancient La Plata. South- 

 ward and seaward of Buenos Ayres, the plains were 

 upheaved from under water inhabited by true marine 

 shells. We further know from Professor Ehrenberg's 

 examination of the twenty microscopical organisms in 

 the mud round the tooth of the Mastodon high up the 

 course of the Parana, that the bottom-most part of 

 this formation was of brackish-water origin. A similar 

 conclusion must be extended to the beds of like com- 

 position, at the level of the sea and under it, at 

 M. Hermoso in Bahia Blanca. Dr. Carpenter finds 

 that the harder varieties of tosca-rock, collected chiefly 

 to the south, contain marine spongoid bodies, minute 

 fragments of shells, corals, and Polvthalamia ; these 

 perhaps may have been drifted inwards by the tides, 

 from the more open parts of the sea. The absence of 

 shells, throughout this deposit, with the exception of 

 the uppermost layers near Buenos Ayres, is a remark- 

 able fact : can it be explained by the brackish condition 

 of the water, or by the deep mud at the bottom ? I 

 have stated that both the reddish mud and the concre- 

 tions of tosca-rock are often penetrated by minute, 

 linear cavities, such as frequently may be observed in 

 fresh-water calcareous deposits : — were they produced 

 by the burrowing of small worms ? Only on this view 



