256 J^otes on JVatural History. [May, 



as often to be passed by an individual without being perceived; 

 upon being disturbed, however, it speedily buries itself in the 

 ground and as quickly becomes lost to the sight. 



The above enumerated animals, with some dozen species of 

 mice, are those most commonly to be met with by a traveller 

 journeying over these plains. 



These Pampas extend into the interior almost to the base of 

 the Cordilleras of the Andes, a distance of nearly three hundred 

 miles, rising up in a succession of terraces, and terminating ab- 

 ruptly in a direction towards the east, and although the surface 

 presents such a bare and sterile appearance, its geological con- 

 struction is of a highly interesting nature. It is unquestionably 

 a member of the Tertiary period, though the green earth so pro- 

 fusely disseminated throughout some of the lower strata, might 

 readily induce an investigator on first inspection, to assign to it 

 a position among the rocks of the cretaceous group, of the Second- 

 ary series. Its entire thickness at this point is about seventy feet, 

 rising up in a bold precipitous manner directly fronting the sea, 

 and by this means affords to the geologist an admirable opportu- 

 nity for inspecting its various strata. This whole formation rests 

 in an unconformable position upon a thick mass of redish brown 

 porphyry, and in an ascending series, the strata of which it is 

 composed may be described in the following order: 



The first and lowest stratum visible, is composed of a greenish 

 sandstone made up almost entirely of particles of green-earth, 

 associated with fine grains of sand, evidently derived from some 

 trappean rocks. It is about six feet in thickness, and is com- 

 pletely charged with fossils, a few of which have a close alliance 

 to the recent species along the shores of the sea. 



The second is a layer of about eight feet thick, composed of a 

 fine grained yellowish marly clay, with but few fossils. 



The next in order is a coarse grained, greenish sandstone, very 

 similar in appearance to the lowest stratum, and is likewise abun- 

 dant in organic remains. It has a thickness of nearly nine feet. 



The fourth in number is a layer of yellowish marly clay, eight 

 feet in thickness, and with rarely a fossil. 



The fifth is a dark bluish sand, partially indurated, though 

 crumbling readily in the hands. It is six feet thick with no re- 

 mains visible. 



The sixth is a fine grained, yellowish, marly clay or marl, ten 

 feet in thickness and abounding with large pectens almost to the 

 exclusion of all other fossils. 



A light bluish covered sandstone succeeds, six feet thick, with 

 some few organic remains. 



The eighth is a ten foot stratum, of a red and yellowish appear- 



