520 Section of the Uspallata Range. part n. 



clay-slate formation : the slate is generally hard and 

 bluish, with the laminae coated by minute micaceous 

 scales : it alternates manv times with a coarse-grained, 

 greenish grauwacke, containing rounded fragments of 

 quartz and bits of slate in a slightly calcareous basis. 

 The slate in the upper part generally becomes purplish, 

 and the cleavage so irregular that the whole consists 

 of mere splinters. Transverse veins of quartz are 

 numerous. At the Calera. some leaonies distant, there 

 is a dark crystalline limestone, apparently included in 

 this formation. With the exception of the grauwacke 

 being here more abundant, and the clay-slate less altered, 

 this formation closely resembles that unconformably 

 underlying the porphyries at the western foot of this 

 same range; and likewise that alternating with the 

 porphyritic conglomerate in the main Cordillera. This 

 formation is a considerable one, and extends several 

 leagues southward to near Mendoza : the mountains 

 composed of it rise to a height of about 2,000 feet 

 above the edge of the Pampas, or about 7,000 feet 

 above the sea. 1 



Secondly : the most usual bed on the clay-slate is 

 a coarse, white, slightly calcareous conglomerate, of no 

 great thickness, including broken crystals of feldspar, 

 grains of quartz, and numerous pebbles of brecciated 

 clay-stone porphyry, but without any pebbles of the 

 underlvinD* clav-slate. I nowhere saw the actual nunc- 

 tion between this bed and the clay-slate, though I spent 

 a whole day in endeavouring to discover their relations. 

 In some places I distinctly saw the white conglomerate 

 and overlying beds inclined at from 25° to 30° to the 

 west, and at the bottom of the same mountain, the 

 clay-slate and grauwacke inclined to the same point, but 



1 I infer this from the height of V. Vicencio, which was ascer- 

 tained by Mr. Miers to be 5,328 feet above the sea. 



