434 La Plata. pake n. 



Southern La Plata. — The first ridge, south of the 

 Plata, which projects through the Pampean formation, 

 is the Sierra Tapalguen and Yulcan, situated 200 miles 

 southward of the district just described. This ridge is 

 only a few hundred feet in height, and runs from C. 

 Corrientes in a WXTT. line for at least 150 miles into 

 the interior : at Tapalguen, it is composed of unstrati- 

 fied granular quartz, remarkable from forming tabular 

 masses and small plains, surrounded by precipitous cliffs : 

 other parts of the range are said to consist of granite : 

 and marble is found at the S. Tinta. It appears from 

 M. Parchappe's l observations, that at Tandil there is a 

 range of quartzose gneiss, very like the rocks of the S. 

 Laro-a near Maldonado, running in the same XXE. and 

 SSYT. direction : so that the framework of the country 

 here is very similar to that on the northern shore of 

 the Plata. 



The Sierra Guitru-gueyu is situated sixty miles 

 south of the S. Tapalguen : it consists of numerous 

 parallel, sometimes blended together ridges, about 

 twenty-three miles in width, and 500 feet in height 

 above the plain, and extending in a NW. and SE. 

 direction. Skirting round the extreme SE. termination, 

 I ascended only a few points, which were composed of 

 a fine-grained gneiss, almost composed of feldspar with 

 a little mica, and passing in the upper parts of the 

 hills into a rather compact purplish-clay state. The 

 cleavage was nearly vertical, striking in a 1SJW- by W. 

 and SE. by E. line, nearly, though not quite, coinci- 

 dent with the direction of the parallel ridges. 



The Sierra Yentana lies close south of that of Guitru- 

 gueyu ; it is remarkable from attaining a height, very 



1 ML d'Orbigny's ' Voyage, Part. G-eolog.' p. 46. I have given a 

 short account of the peculiar forms of the quartz hills of Tapalguen, 

 so unusual in a metamorphic formation, in my ' Journal of Ke« 

 searches' (2nd edit.), p. 116. 



