TRANSACTIONS OF SECTJON E. 927 



trausported to the plain, to the east of Santa Cruz de la Sierra, millions on millions 

 of cubic yards of material, and have filled it to a depth of several hundred feet. 

 The examples I have given from personal observation can, however, but faintly 

 convey an idea of the grand scale upon which Nature is at work tearing down the 

 Atlantic slope of the Bolivian Andes. So far, she has succeeded in only outlining 

 the task she has assumed, for she appears to have finished nothing. The finer 

 touches cannot be put on with such riotous vigour. 



Later, I shall show how the closing of the southern gap caused the formation 

 of a lake in the Bolivian Mqjos basin, the lacustrine character of which is not vet 

 eliminated ; for, during a period of about four months of the year, some 35,000 

 square miles of its surface are covered with the surplus water which cannot find 

 exit over the falls of the Madeira, they not yet having been worn down enough to 

 give complete drainage to the basin ; or else this is not yet sufficiently silted up to 

 keep it out of water. Castlenau says that, ' Due to their horizon tality, all the 

 plains, from the mouth of the Mamor^ to the Pilcomayo, are inundated from 

 October to March, and present the aspect of a great ocean dotted with green 

 islands ; ' and, speaking of the great southern gap, says : ' Across the Monte Grande, 

 a simply overturned tree would change the course of the waters.' D'Orbigny is 

 eloquent in his descriptions of the ' smooth surface ' and ' unlimited horizon ' of the 

 vast plains of Mojos and the Beni. 



Between Santa Cruz de la Sierra and the northern frontier of the Argentine 

 Republic, the Pilcomayo Eiver gathers its waters amidst masses of red sandstone 

 and argillaceous schists. Further southward, the Andes are broken into a number 

 of secondary ridges of palaeozoic composition, among which are the sources of the 

 Bermejo and Salado. In the most southern extension of these ridges we fijid 

 limestones and compact sandstones, mica schist, gneiss, and granite. From about 

 latitude 30° south the Andes lose their great width, and thence confine themselves 

 to the Pacific coast ridge, as far south as the Straits of Magellan. According to 

 De Moussy, ' the lower sierras, which lie to the east, present a great variety of 

 stratified, crystalline rocks, saccharoids, slate schists, bituminous sandstone, 

 basalts, obsidian, trachites, pumice, crystallised and amorphous quartz.' 



The Sierra de Cordova, extending north and south for nearly 300 miles, and 

 having a summit about 7,500 feet above ocean-level, was probably an ancient group 

 of islands in the Pampean Sea. It consists of several parallel chains, composed 

 principally of quartz, gneiss, and limestones of various colours. De Moussy found 

 trachitic rocks on their northern extension, and evidences of volcanic action. 

 South-west, and separated from the Sierra de Cordova, is the low San Luis range 

 of gneiss and mica schist, and sometimes crystallised quartz. Vast masses of 

 rounded shingle, covered by a thin cap of argillaceous earth and coarse granitic 

 sand, border the valleys. The Alto Penasco sierra, forty miles west of the San Luis 

 range, is also composed of cr3'stalline rocks. The Cordon de Paramillos, near 

 Mendoza, is of porphyry, sandstone, schists, and limestone. Here also, ' immense 

 quantities of rounded and rolled shingle cover the base of all the chains: and 

 interior valleys. The torrents cut the accumulated debris to a profound depth, and 

 expose its enormous thickness. The bottoms of the valleys are entirely composed 

 of it.' 



Penetrating north of Mendoza, we find white sandstone, and mountains of red 

 sandstone and argillaceous conglomerate in full process of decomposition. These 

 abound, above all, in the provinces of La Rioja and Catamarca. 



Southern Extremity of the Basin. — From Cape Corrientes inland, ranges of hills, 

 irregularly massed in line, extend north-west about 150 miles. They are known 

 as the Sierra de Tandil, and are widest at Tandil. Their greatest elevation above 

 the sea is 1,470 feet. My old friends Ileusser and Claraz say the range is ' com- 

 posed of sandstone caps resting on gneissic-granite, showing the Pampean forma- 

 tion in the valleys and on their slopes.' Sometimes the gneissic-granite shows 

 bare, and at others elevated into sierras with slopes covered with Pampean 

 formation. 



Further south-west, and lying between the Tandil and Bahia Blanca, are the 

 metamorphic sierras Pillahuinco, Ventana, and Curamalal, extending west and 



