Fknton — Physiograjjhj and Glacial Geology of S. Patagonia. 205 



conditions exist ac present, no great erosion is taking place in the river 

 valleys; and it is quite possible, as far as we can see, that the beds of the 

 rivers stood practically at the same level many thousands of years ago as 

 they do to-day. The hilltops and mountains to the west are, of course, 

 becoming worn away, but the lowlands and valleys, wherever the surface is 

 protected by vegetation, are probably, if anything, rising. I have found 

 abundance of evidence of this latter fact in many places, in the shape of 

 bones of recent animals, such as guanaco, &c., some feet below the surface of 

 tlie soil in the valleys. If we dig down through any of the river valleys, 

 some distance up from the sea, we pass through a considerable depth of 

 shingle and sand, very similar to what we find on the pampa, perhaps fifteen 

 or twenty feet before we come on the actual tertiary rock-floor. We also 

 find in places a considerable depth of mud and black soil where the gradient 

 of the valley is slight ; in a tributary dry canadon, where the gradient is 

 more marked, there is not as a rule much black soil or mud ; but even here 

 the floor of the caiiadon is covered by a layer of, perhaps, fifteen to twenty 



feet of sandy shingle. In very few cases — and this is a very important fact 



is the tertiary rock-floor of the canadon exposed to view, the only exceptions 

 being those cafiadones which have very steep gradients, and which drain 

 very extensive areas. All the river valleys and cafiadones in southern 

 Patagonia, with the exception of the Santa Cruz river, are subject to occa- 

 sional spring floods. Those floods vary in different localities, and indifferent 

 years, but they are always found at times right through the country. 



The Santa Cruz river is an exception, as it does not begin to rise until the 

 summer, and does not reach its height till well into the autumn. The reason 

 for this is that this river takes its origin in some large lakes situated well in 

 between the Cordilleras, and, as the snow on the latter melts much later than 

 that which falls on the plains, the floods are consequently delayed. Most of 

 the terminal branches of the Gallegos river rise in the plains ; one or two of 

 them, however, extend as far as the Cordilleras, and consequently occasional 

 summer or even autumn floods occur in the Gallegos river after periods of 

 hot weather, when additional snow has melted on the mountains. The local 

 rainfall has very little effect on the floods in this country. Even after several 

 days of heavy rain, such as occur occasionally, although the small streams 

 may be slightly increased in amount, the rivers show practically no rise. 

 Eain never fills up the lagoons, and experienced farmers will tell you that 

 without snow they never have their land properly supplied with water. Any 

 rain falling on the shingle-surface of the country will sink in unless the 

 surface is made water-tight by being frozen. Now, when the snow begins to 

 melt in the spring, the superficial layer of the soil is actually frozen, so that 



SOIENT. PKOC. E.D.S., VOL. XVI, NO. XIX. 2 H 



