The humid, grassy, pampean country extends, roughly speaking, half-way from the Atlantic Ocean and the Plata and Paraná rivers to the Andes, and passes gradually into the "Monte Formation," or _sterile pampa_--a sandy, more or less barren district, producing a dry, harsh, ligneous vegetation, principally thorny bushes and low trees, of which the chañar (Gurliaca decorticans) is the most common; hence the name of "Chañar-steppe" used by some writers: and this formation extends southwards down into Patagonia. Scientists have not yet been able to explain why the pampas, with a humid climate, and a soil exceedingly rich, have produced nothing but grass, while the dry, sterile territories on their north, west, and south borders have an arborescent vegetation. Darwin's conjecture that the extreme violence of the _pampero,_ or south-west wind, prevented trees from growing, is now proved to have been ill-founded since the introduction of the Eucalyptus globulus; for this noble tree attains to an extraordinary height on the pampas, and exhibits there a luxuriance of foliage never seen in Australia. To this level area--my "parish of Selborne," or, at all events, a goodly portion of it--with the sea on one hand, and on the other the practically infinite expanse of grassy desert--another sea, not "in vast fluctuations fixed," but in comparative calm--I should like to conduct the reader in imagination: a country all the easier to be imagined on account of the absence of mountains, woods, lakes, and rivers. There is, indeed, little to be imagined--not even a sense of vastness; and Darwin, touching on this point, in the _Journal of a Naturalist,_ aptly says:--"At sea, a person's eye being six feet above the surface of the water, his horizon is two miles and four-fifths distant. In like manner, the more level the plain, the more nearly does the horizon approach within these narrow limits; and this, in my opinion, entirely destroys the grandeur which one would have imagined that a vast plain would have possessed." I remember my first experience of a hill, after having been always shut within "these narrow limits." It was one of the range of sierras near Cape Corrientes, and not above eight hundred feet high; yet, when I had gained the summit, I was amazed at the vastness of the earth, as it appeared to me from that modest elevation. Persons born and bred on the pampas, when they first visit a mountainous district, frequently experience a sensation as of "a ball in the throat" which seems to prevent free respiration.