350 Pampean Formation. paet n. 



of the continent : it is known that in Brazil an immense 

 area consists of gneissic rocks, and we shall hereafter 

 see, over how great a length the plutonic rocks of the 

 Cordillera, the overlying purple porphyries, and the 

 trachytic ejections, are almost identical in nature. 



Three theories on the origin of the Pampean forma- 

 tion have been propounded : — First, that of a great 

 debacle by M. d'Orbigny ; this seems founded chiefly 

 on the absence of stratification, and on the number of 

 embedded remains of terrestrial quadrupeds. Although 

 the Pampean formation (like so many argillaceous 

 deposits) is not divided into distinct and separate strata, 

 yet we have seen that in one good section it was striped 

 with horizontal zones of colour, and that in several 

 specified places the upper and lower parts differed, not 

 only considerably in colour, but greatly in constitution. 

 In the southern part of the Pampas the upper mass (to 

 a certain extent stratified) generally consists of hard 

 tosca-rock, and the lower part of red Pampean mud, 

 often itself divided into two or more masses, varving in 

 colour and in the quantity of included calcareous matter. 

 In western Banda Oriental, beds of a similar nature, 

 but of a greater age, conformably underlie and are 

 intercalated with the regularly stratified tertiary forma- 

 tion. As a general rule, the marly coucretions are 

 arranged in horizontal lines, sometimes united into 

 irregular strata : surelv, if the mud had been tUmul- 

 tuously deposited in mass, the included calcareous 

 matter would have segregated itself irregularlv, and 

 not into nodules arranged in horizontal lines, one above 

 the other and often far apart : this arrangement appears 

 to me to prove that mud, differing slightly in composi- 

 tion, was successively and quietly deposited. On the 

 theory of a debacle, a prodigious amount of mud, with- 

 out a single pebble, is supposed to have been borne over 



