All pastures are not equally available in every season. Those which are at a height of more than 4,000 feet in the north, and 2,300 to 2,600 feet in the south, are covered in winter with a thick mantle of snow. These are summer pastures. During the winter the animals are brought down to the principal valleys or to sheltered _canadones_ below the level of the tableland. The _mallin_ is, as a rule, a winter pasture. When it is too wet, however, it is treacherous, and the animals are buried in it. They have to wait for fine weather before going into it. The pastures, too, which have no permanent water supply, or have only _manantiales_, which dry up at the beginning of summer, can only be used during the winter. Hence each ranch has to have, besides its assured water supply, a suitable combination of summer and winter pasturage, and it is far from certain that this will be found on every estate, cut up geometrically for colonization, as they were, by the administration of lands. [Illustration: THE VOLCANO PUNTIAGUDO. _On the Chilean side, to the north of the road from Lake Nahuel Huapi. The glaciers come down lower on the western side, as the moist winds come from the west, and the rain becomes less and less frequent as one goes eastward toward the Patagonian tableland._] [Illustration: CERCAS ON THE LIMAY (RISING IN LAKE NAHUEL HUAPI), NEAR THE CONFLUENCE OF THE TRAFUL. _Here the Limay enters the sub-desert tableland. Last trees (cypresses) in the valley in the foreground._ Photograph by Bailey Willis. PLATE XIII. To face p. 142.] The constitution of the flock and the first occupation of the land have compelled breeders to undertake difficult journeys, and more than one of these proved disastrous. The earliest arrivals, driving their sheep along little-known tracks, could not avoid losses in crossing the arid parts of the tableland: parts which D'Orbigny, translating literally the Spanish word _travesia_, calls "crossings."[70] When the ranch is established, the breeding does not necessitate any further movements of the flocks to a great distance, apart from certain special migrations, or "transhumations," which I will consider later. It is on each ranch, sometimes on each group of ranches combined in a single estate, that they pass alternately from winter to summer pasture. The only transport necessary is that of wool. The fleeces, which the west wind has heavily laden with dust, are collected in the sheds belonging to the ranch, or, in the case of the _intrusos_, on the premises of certain small traders (_bolicheros_) who are scattered over the tableland even at its extreme limits. Convoys of wagons then take them to the ports on the coast. [70] The search for possible routes for cattle in the districts that were not yet colonized helped in the study of Patagonia. Moyano was doing this when he explored the route from Santa Cruz to Lake Nahuel Huapi.