THE SCRUB OF THE PAMPA 163 



the right bank, at San Lorenzo, This is the domain 

 of the ombu, a tree with thick trunk and naked roots 

 which is found scattered over the prairie in the Parana 

 region as far as south of Buenos Aires. 



In the west, between San Luis and the mouth of 

 the Colorado, the transition from the Pampa to the 

 monte is gradual. Just as at Santa Fe, the approach 

 of the monte is announced by the appearance of chanares, 

 in the south-west corner of the Cordoba province 

 and on the southern slope of the Sierra de la Ventana. 

 The monte, properly so called, though impoverished, 

 invaded by the jarilla, and mainly composed (as in 

 northern Patagonia) of dwarf mimosas, covers the 

 area of the Pampean sierras on the left bank of the 

 Chadi Leuvu and the Colorado. Between this area 

 and a line passing through Rancul, Anguil, Atreuco, and 

 Bernasconi, where the naked prairie begins, there is 

 a mixed zone which one may call the calden zone. 

 This mimosa, a near relative of the algarroba, which 

 has a wider range than the other plants of the monte 

 in this latitude, forms woods at intervals in the south 

 of the San Luis province and on the flanks of the 

 parallel valleys of the central Pampa. Between these 

 woods the tableland is generally covered by the prairie, 

 with occasional patches of chanares. About twenty-five 

 miles east of Buena Esperanza the line from San Rafael 

 touches the far corner of a forest of caldenes, which 

 stretches south-westward, and reaches the Rio Salado 

 about 35° 30' S. lat. Beyond Buena Esperanza it 

 keeps on the prairie as far as the crossing of the Salado, 

 which here marks the limit of the monte. The Rio Negro 

 line passes directly from the prairie to the Patagonian 

 scrub mid-way between Bahia Blanca and the Colorado. 



Within these limits the prairie extends without a 

 break. The sierras of the Buenos Aires province have 

 no arborescent vegetation. 



The zone of the prairie, intermediate between tropi- 

 cal Argentina and the sub-desert regions of western 



