3 1 4 Pampean Formation. paet n. 



times stalactiformed ; they are of a compact structure, 

 but often penetrated (as well as the mud) by hair-like 

 serpentine cavities, and occasionally with irregular 

 .fissures in their centres, lined with minute crystals of 

 carbonate of lime ; they are of white, brown, or pale 

 pinkish tints, often marked by black dendritic man- 

 ganese or iron ; they are either darker or lighter tinted 

 than the surroundino; mass : thev contain much car- 

 bonate of lime, but exhale a strxmg aluminious odour, 

 and leave, when dissolved in acids, a larsre but varying 

 residue, of which the greater part consists of sand. 

 These concretions often unite into irregular strata ; and 

 over very large tracts of country, the entire mass consists 

 of a hard, but generally cavernous marly rock : some 

 of the varieties might be called calcareous tuffs. 



Dr. Carpenter has kindly examined under the mi- 

 croscope, sliced and polished specimens of these concre- 

 tions, and of the solid marl-rock, collected in various 

 places between the Colorado and St. Fe Bajada. In 

 the greater number, Dr. Carpenter finds that the whole 

 substance presents a tolerably uniform amorphous cha- 

 racter, but with traces of incipient crystalline meta- 

 morphosis ; in other specimens he finds microscopically 

 minute rounded concretions of an amorphous substance 

 (resembling in size those in oolitic rocks, but not having 

 a concentric structure), united by a cement which is 

 often crystalline. In some. Dr. Carpenter can perceive 

 distinct traces of shells, corals, Polythalamia, and rarely 

 of spongoid bodies. For the sake of comparison, I sent 

 Dr. Carpenter specimens of the calcareous rock, formed 

 chieflv of fragments of recent shells, from Coquimbo in 

 Chile : in one of these specimens, Dr. Carpenter finds, 

 besides the larger fragments, microscopical particles of 

 shells, and a varying quantity of opaque amorphous 

 matter ; in another specimen from the same bed, he 



