THE PATAGONIAN STEPPE. 365 



The Patagonian Steppe Lands. 



The interior of the Patagonian districts watered by the Rios Colorado and 

 Negro bristles with rocky crests, pink porphyries, and granites, which look in the 

 morning sun like lightly-tinted vapours. These various groups, known by the 

 general name of mahulda, that is, "mountains" in the native language, have an 

 average height of from 1,300 to 1,650 feet, and are nearly all disposed north- 

 west and south-east, like the chains of hills between Buenos Ayres and Bahia, 

 Blanca. 



Between the ridges the ground is strewn with rolled pebbles, granites, gneiss, 

 porphyries disposed in horizontal layers alternating with the dunes. These beds 

 of rolled gravels cover all the tertiary plains which constitute the whole of the 

 Patagonian plateau east of the Andes, and which contain a superabundance of 

 fossil remains. This prodigious mass of Patagonian gravels was calculated by 

 Darwin to extend for about 600 miles north and south, with a mean breadth of 

 200 miles, and a depth of 50 feet. Whole mountain ranges must have been 

 triturated to yield such gravel beds as these ; and to the Patagonian deposits must 

 be added the detritus of like nature at present covering the marine bed itself. 

 Such are the rolled porphyries which the soundings have fished up in the waters 

 of the Falkland Islands, far from any insular masses containing analogous rocks. 



These pebbles are evidently derived from the Andes and the older mountains, 

 which formerly rose above the central and eastern plains, and of which nothing 

 now remains except the nuclei. Glacial moraines have undoubtedly supplied the 

 raw material, which has been distributed by the marine waters in horizontal or 

 very slightly inclined beds. Then followed the phenomenon of emersion, due 

 either to an upheaval of the land, or to a subsidence of the sea. Thus the old 

 shingly beach became the dry gravel pits of Patagonia, in which are found pro- 

 digious quantities of those gigantic oysters, 15 to '^0 inches round, which are so 

 widely diffused throughout the soil of Patagonia, Near Possession Bay, at the 

 Atlantic entrance of Magellan Strait, de Pourtalès discovered a lagoon standing 

 160 feet above sea level, and containing shells absolutely identical with those of 

 the neighbouring waters. 



Hence there can be no reasonable doubt as to the general movement of 

 upheaval along the Patagonian seaboard. But geologists have not yet determined 

 its true character, and while some suppose that it took place in a succession of 

 sudden upward thrusts, corresponding to the several raised terraces, others with 

 more probability suggest that it was, on the contrary, a slow movement produced 

 in a series of rhythmical undulations. 



During the contemporary period other formations have been superimposed on 

 the Patiigonian gravel beds, and on the argillaceous clays of Central Argentina. 

 Over vast spaces the ground is now covered with sands, Avhich form dunes 

 analogous to those developed on many coastlands under the influence of the winds 

 setting from the high seas. But in the Plateau regions these shifting dunes are 

 not of marine origin ; they are, on the contrary, derived from the region of foot- 



