chap. xi. Banda Oriental'. 343 



and in the lower part into soft white sandstone, and 

 even into loose sand : beneath this sandstone, I saw 

 in two places layers of calcareous and marly rocks, and 

 in one place red Pampean-like earth ; at the base of 

 these sections, there was a hard, stratified, white sand- 

 stone, with chalcedonic layers. Near Mercedes, beds 

 of the same nature and apparently of the same age, are 

 associated with compact, white, crystalline limestone, 

 including much botryoidal agate, and singular masses, 

 like porcelain, but really composed of a calcareo-sili- 

 ceous paste. In sinking wells in this district the 

 chalcedonic strata seem to be the lowest. Beds, such 

 as here described, occur over the whole of this neigh- 

 bourhood ; but twenty miles further up the R. Negro, 

 in the cliffs of Perika, which are about fifty feet in 

 height, the upper bed is a prettily variegated chalce- 

 dony, mingled with a pure white tallowy limestone; 

 beneath this there is a conglomerate of quartz and 

 granite ; beneath this many sandstones, some highly 

 calcareous ; and the whole lower two-thirds of the cliff 

 consist of earthy calcareous beds of various degrees of 

 purity, with one layer of reddish Pampean-like mud. 



When examining the agates, the chalcedonic and 

 jaspery rocks, some of the limestones, and even the 

 bright red sandstones, I was forcibly struck with 

 their resemblance to deposits formed in the neigh- 

 bourhood of volcanic action. I now find that M. 

 Isabelle, in his \ Voyage a Buenos Ayres,' has described 

 closely similar beds on Itaquy and Ibicuy (which enter 

 the Uruguay some way north of theR. Negro) and these 

 beds include fragments of red decomposed true scorias 

 hardened by zeolite, and of black retinite : we have 

 then here good evidence of volcanic action during our 

 tertiary period. Still farther north, near S. Anua, ! 

 1 M. d'Orbigny, ' Voyage, Fart. Geolog.,' p. 29. 



