chap. xi. Pampean Formation. 315 



finds the whole composed of the amorphous matter, 

 with layers showing indications of an incipient crystal- 

 line metamorphosis : hence these latter specimens, both 

 in external appearance and in microscopical structure, 

 closely resemble those of the Pampas. Dr. Carpenter 

 informs me that it is well known that chemical preci- 

 pitation throws down carbonate of lime in the opaque 

 amorphous state ; and he is inclined to believe that the 

 long-continued attrition of a calcareous body in a state 

 of crystalline or semi-crystalline aggregation (as, for 

 instance, in the ordinary shells of Mollusca, which, 

 when sliced, are transparent) may yield the same result. 

 From the intimate relation between all the Ooquimbo 

 specimens, I can hardly doubt that the amorphous 

 carbonate of lime in them has resulted from the attri- 

 tion and decay of the larger fragments of shell : whether 

 the amorphous matter in the marly rocks of the Pampas 

 has likewise thus originated, it would be hazardous to 

 conjecture. 



Por convenience sake, I will call the marly rock by 

 the name given to it by the inhabitants, namely, Tosca- 

 rock ; and the reddish argillaceous earth, Pampean 

 mud. This latter substance, I may mention, has been 

 examined for me by Professor Ehrenberg, and the 

 result of his examination will be given under the proper 

 localities. 



I will commence my descriptions at a central spot, 

 namely, at Buenos Ayres, and thence proceed first south- 

 ward to the extreme limit of the deposit, and afterwards 

 northward. The plain on which Buenos Ayres stands 

 is from thirty to forty feet in height. The Pampean 

 mud is here of a rather pale colour, and includes small 

 nearly white nodules, and other irregular strata of an 

 unusually arenaceous variety of tosca-rock. In a well at 

 the depth of seventy feet, according to Ignatio Nunez, 



