82 BXPLOKING EXPEDITION FROM SANTA Ffi 



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

 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 the val- 

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

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

 yellow foliated sandstones or sandy shales, 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 Cretaceous fossil, Ammonites pla- 

 centa, and in lenticular masses of limestone immense numbers of BacuUtcs aitct')>x, ( 1 nr- 

 (liiun lellalum, Meek, Astarte Shumanli, M., Aporrhais Ncwlern/i, Meek, &c. 



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

 (Nos. 4 and 5 of Meek and Hayden's section of the Cretaceous 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 Baculitcs are so numer- 

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

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

 maybe identical. The smooth ones resemble //. ovatus and II. coiitpressus. They 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 fine 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 the 

 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 clay-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 Rro 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 Indians of the existence 

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



