n 



1 6 Pampean Formation. paet n. 



mucli tosca-rock was met with, and at several points, 

 at 100 feet deep, beds of sand have been found. I have 

 already given a list of the recent marine and estuary 

 shells found in many parts on the surface near Buenos 

 Ayres, as far as three and four leagues from the Plata. 

 Specimens from near Ensenada, given me by Sir W. 

 Parish, where the rock is quarried just beneath the 

 surface of the plain, consist of broken bivalves, cemented 

 by and converted into, white crystalline carbonate of 

 lime. I have already alluded, in the first chapter, to 

 a specimen (also given me by Sir W. Parish) from the 

 A. del Tristan, in which shells, resembling in every 

 respect the Azara labiata, d'Orbig., as far as their 

 worn condition permits of comparison, are embedded in 

 a reddish, soi'tish, somewhat arenaceous marly rock : 

 after careful comparison, with the aid of a microscope 

 and acids, I can perceive no difference between the 

 basis of this rock and the specimens collected by me 

 in many parts of the Pampas. I have also stated, on 

 the authority of Sir W. Parish, that northward of 

 Buenos A3n*es, on the highest parts of the plain, about 

 forty feet above the Plata, and two or three miles from 

 it, numerous shells of the Azara labiata (and I believe 

 of Venus sinuosa) occur embedded in a stratified 

 earthy mass, including small marly concretions and said 

 to be precisely like the great Pampean deposit. Hence 

 we may conclude that the mud of the Pampas continued 

 to be deposited to within the period of this existing 

 estuary shell. Although this formation is of such 

 immense extent, I know of no other instance of the 

 presence of shells in it. 



Buenos Ayres to the Rio Colorado. — With the 

 exception of a few metamorphic ridges, the country 

 between these two points, a distance of 400 geographical 

 miles, belongs to the Pampean formation, and in the 



