chap. xi. Sierra Ventana. 319 



Spanish officers l ) 840 feet above the sea-level. On the 

 flanks of the mountains, at a height of 300 or 400 feet 

 above the plain, there were a few small patches of con- 

 glomerate and breccia, firmly cemented by ferruginous 

 matter, to the abrupt and battered face of the quartz- 

 traces being thus exhibited of ancient sea-action. The 

 high plain round this range sinks quite insensibly to 

 the eye on all sides, except to the north, where its 

 surface is broken into low cliffs. Round the Sierras 

 Tapalguen, Guitru-gueyu, and between the latter 

 and the Ventana we have seen (and shall hereafter 

 see round some hills in Banda Oriental) ; that the 

 tosca-rock forms low, flat-topped, cliff-bounded hills, 

 higher than the surrounding plains of similar compo- 

 sition. From the horizontal stratification and from 

 the appearance of the broken cliffs, the greater height 

 of the Pampean formation round these primary hills 

 ought not to be altogether or in chief part attributed 

 to these several points having been uplifted more ener- 

 getically than the surrounding country, but to the 

 argillaceo-calcareous mud. having collected round them, 

 when they existed as islets or submarine rocks, at a 

 greater height, than at the bottom of the adjoining 

 open sea ; — the cliffs having been subsequently worn 

 during the elevation of the whole country in mass. 



Southward of the Ventana, the plain extends farther 

 than the eye can range; its surface is not very level, 

 having slight depressions with no drainage exits ; it is 

 generally covered by a few feet in thickness of sandy 

 earth ; and in some places, according to M. Parchappe, 2 

 by beds of clay two yards thick. On the banks of the 

 Sauce, four leagues SE. of the Ventana, there is an im- 

 perfect section about 200 feet in height, displaying in 

 the upper part tosca-rock and in the lower part red 



1 < La Plata,' &c. by Sir W. Parish, p. 146. 



2 M. d'Orbigny, ' Voyage, Part. Geolog.' pp. 47, 48. 



