Fenton — Physiography and Glacial Geology of S. Patagonia. 191 



erosive action that cut out the river-valleys and canadones. In oire place the 

 top of one of these hills was found to be 520 feet above the river, and this was 

 the same as the level of the nearest high pampa. 



The next question in the study of the pampa is the shingle, which, as 

 C. Darwin and others long ago pointed out, is found practically all over the 

 plains of Southern Patagonia. In fact, I do not believe tliere is a single 

 square yard of the high pampa between the Straits of Magellan and the Santa 

 Cruz river without its shingle covering. The shingle forms a comparatively 

 thin layer, varying from 10 to 25 feet in thickness. It lies in practically 

 every instance on well-stratified middle tertiary rock, the latter in most cases 

 consisting of a more or less whitish, greyish, or dull blue soft stone, locally 

 called Tosca. This peculiar formation, although apparently sedimentary, has 

 yielded nothing but mammalian and bird fossils. The pebbles on the surface 

 are as a rule broken and angular, as the result of repeated pampa fires ; but 

 below the surface they are comparatively well rounded and sufficiently worn 

 to show that they have been subjected to prolonged water action. 



The diversity in composition of the individual stones which make up this 

 shingle is remarkable. Some are white, some are black, some are yellow, 

 some are red, and some are green ; but by far the greater number in most 

 localities consists of a yellowish porphyritic variety. No particular bedding, 

 either false or true, is to be seen in any of these layers of shingle ; in fact, 

 they give one the idea of ballast dumped higgledy-piggledy out of wagons to 

 till up the place. This same absence of arrangement of the stones is to be 

 found in almost all parts of the first pampa. The stones found on the pampa 

 are often as much as six to eight inches in length, and we might call the 

 greater number of them three-quarters rounded, many of the stones still 

 exhibiting the original sides, and having only the corners smoothed off. 



I would once more wish to impress on the reader that one of the most 

 extraordinary characteristics of this shingle is its universal and even distri- 

 bution ; it is spread broadcast, practically without a break, over almost the 

 whole surface of Argentine Patagonia. It is found on the highest hills, in 

 the lowest valleys, and even extends, as has been shown by the Beagle 

 expedition, for hundreds of miles into the Atlantic under the sea. It never 

 occurs in heaps, and, as seen in the cliff sections, the underlying rock shows, 

 when completely covered by this shingle, practically no indentation or 

 cuttings — in fact, it exhibits the same wonderful levelness that the over- 

 hanging shingle does. The unconformity between the two is apparently very 

 slight, and yet there must have been a very considerable hiatus in time 

 between the depositions of the two formations. 



As we trace the face of a cliff down on the Atlantic coast, there is no 



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