204 Scientific Froccedivris^ Uoyal Dublin Society 



wide, in some places more, in some less, and the surface is practically level, 

 no portion of it being more tLa.n ten to fifteen feet above the liver. The 

 sides of this valley are bounded by steep rising banks, probably eighty feet 

 in height, which mark the beginnings of the first terraces. The floor is in 

 most places covered with a rich black soil, on which grows an abundance of 

 rank vegetation, except in these places which have been eaten down from 

 over-stocking with animals. Small lakes occur here and there, and the river 

 winds about through the valley in a very sluggish manner, now on one side 

 and now on the other. Patches of swamp lie in different directions, yet in 

 no place is the river very deep ; in fact, almost every half mile or so it is 

 capable of being forded. The river is always shifting its position through 

 the valley, and as it cuts a new course the old one becomes first a lagoon, 

 afterwards a swamp, and finally becomes obliterated by vegetation, mud, and 

 various kinds of rubbish which have fallen in. The river during the ordinary 

 dry weather in summer runs between perpendicular banks, is perhaps about 

 five feet below the level of the main valley, and is anything from fifty to a 

 hundred yards in breadth. If, however, we visit it during the month of 

 September, which is the spring of the year in Patagonia, we shall probably 

 find that it has risen considerably, possibly flowing over the banks in places 

 and flooding parts of the valley. These spring floods are a marked feature 

 in this country ; they \'ary much in different years and in different rivers, 

 being sometimes hardly perceptible, while at other times they fill the whole 

 valley from side to side with one great river. Seven or eight years may pass 

 during which there are practically no floods or only very slight ones, and 

 this period may be followed by one or two years when the whole valley may 

 be filled. When the river rises sufficiently high to flood the whole valley, in 

 about a week or two it usually recedes once more between its banks ; it 

 may, however, remain in a flooded condition in its own channel for more than 

 a month. After the flood has gone down it is surprising how little change 

 has taken place. We may find a few bits of twig deposited here and there 

 along the sides, and some mud in odd places ; but in most localities the 

 valley is as it was before; in fact, if any alteration has taken place, it is that 

 the river valley now contains somewhat more soil than it had formerly. 

 These spring floods are always the result of the melting of the snow on the 

 high lands towards the west, and are, consequently, greatest in those years 

 when there is most snow. Any erosion of the valley during flood-time is 

 more than compensated for by the deposit carried down from above, and, as 

 erosion by wind does not take place on account of the amount of vegetation 

 which protects the surface, we have reason to believe that under the present 

 conditions the level of the valley is slowly rising. At any rate, as climatic 



