82 EXPLORING EXPEDITION PROM SANTA KK 



of snow are visible upon them, which, by their wasting 1 , supply the flow of the Rio de 

 la Plata, which rises in the gorges beneath them." 



The geology of the vicinity is similar to that of much of our route already passed ; 

 the prevailing rocks'are Upper Cretaceous, which compose the hills bordering llic val- 

 ley; the thick mass of sandstone, which forms the Piedra Parada, caps these hills and has 

 a rapid dip away from the mountains: this is succeeded below by a series of gray and 

 yellow foliated sandstones or sandy Bhales, containing immense quantities of fucoidal 

 stems, so many indeed that these casts make up the greater part of the deposit. They 

 are a half-inch in diameter, the surface covered with indistinct annular markings. Be- 

 side the fucoids, these beds contain the well-known (Yetaceous fossil, Ammonites j>la- 



<ei//<(, and in lenticular masses of limestone immense numbers of Baculites anccps, ('in- 

 dium br/hihdi), Meek, Astarte Shumardi, M., Aporrhais Newberryi, .Meek, &c. 



Several of these fossils are common in the Upper Cretaceous beds ot Nebraska 

 (Nos. 1 and 5 of Meek and llavden's section of the ( Yetaceous rocks of Nebraska), 



and afford satisfactory confirmation of the parallelism before suggested between the 



rocks of this region and those of more eastern localities. The Baculites are SO numer- 

 ous as fco form in some places half the bulk of the rock. Of these, the greater num- 

 ber are ornamented with nodes, in the manner of />'. asper (Roemer), with which they 



may be identical. 'The smooth ones resemble II. OVdtus and B. COmpreSSUS. Thev vary 



much in form, some being nearly cylindrical, others much compressed, with every pos- 

 sible variety between these extremes. All, however, as it seems to me, belongs to one 



species. 



From the summit of the hills, near camp, we have a tine view of the country 

 south of us to the San Juan. All this interval is occupied by a mesa composed of the 



Upper Cretaceous rocks, deeply scored along the lines of drainage. The dip of tin; 



rocks composing this plateau is here southward, and very rapid; farther from the bases 

 of the mountains they seem to lie nearly horizontal. 



The southern end of the Sierra de la Plata is composed mainly of light-Colored 

 porphyry and other forms of erupted rock. It also consists in part of granite, with 

 mica and (day-slate, traversed by extensive veins of quartz and epidote, which are 

 metalliferous, containing magnetic iron, and sulphides of iron, copper, and lead ; doubt- 

 less, also, in certain localities, silver and gold; at least there is better promise of the 

 discovery of the precious metals in these mountains than in any others we have visited 

 since leaving the Rio Grande. 



The name given by the Spaniards to this great sierra, Silver Mountain, would 

 seem to indicate that at some time silver had been found there, but I cannot learn 

 that any definite knowledge is possessed by the Mexicans or the [udians of the existence 

 of metallic veins such as would justify their choice of a title so significant 



