Vol. 59.] GEOLOGY OF PATAGONIA. 169 



shown. Not only does it figure as a loose cement, but in places 

 it is further developed as a homogeneous layer some inches thick ; 

 and in a small caiiadon leading to Senor Richmond's estancia I 

 observed a similar deposit of much greater thickness containing bad 

 casts of mollusca, but owing to the amount of talus I could not 

 make sure of its relation to the pebbles. At Monte Leon the 

 matrix is more argillaceous, and contains little of the calcareous 

 cement. Nowhere did I see it in distinct bands. 



The pebbles are, everywhere on the eastern coast and for some 

 distance inland, when in situ and not directly exposed to the atmo- 

 sphere, perfectly rounded. On the pampas in the Territory of Chubut 

 those on the surface retain their rounded form, but on the pampas 

 south of Santa Cruz, where the frosts are more severe, angular pebbles 

 are not uncommon. To see that this is not the original form of the 

 pebbles when deposited it is only necessary to go to the nearest 

 cafiadon in which a section is exposed, where they will be found 

 perfectly rounded. Nowhere do they show ice-scratches, nor does 

 their mode of deposition suggest a glacial, but rather an aqueous 

 origin. Sometimes, as in the valley of the Chico de Chubut, they 

 appear to have been slightly polished by the action of the wind. 



VI. The Origin of the Tehuelche Pebble-Bed. 



The glacial theory of the origin of the Tehuelche Pebble-Bed has 

 not met with by any means general acceptance. M. Mercerat states 

 that only in the river-valleys, where the denuded pebbles are heaped 

 together, could a glacial origin be mistakenly ascribed. 1 Mr. Hatcher 

 shows how the marine beds at Cape Fair weather pass up into the 

 Pebble-Bed imperceptibly. 2 Darwin considered that it was of marine 

 origin. 3 On the other hand, the waterworn appearance of the 

 pebbles cannot alone be taken as a fatal objection to the glacial 

 theory, for Mr. Philip Lake has informed me that certain old 

 moraines in the Himalayas are composed entirely of well-rounded 

 pebbles, without any signs of scratching. That the Pebble-Bed on 

 the eastern coast can owe its origin directly to glacial action is, in my 

 opinion, impossible, since in that case pebbles of basalt, over which 

 any glacier coming to Santa Cruz from the Cordilleras would have 

 to pass, would be mingled with the porphyries. As it is, no one has 

 noted them in any locality near the coast ; but Darwin mentioned 

 their presence some way up the Bio Santa Cruz, due probably 

 to the mingling of the Pebble-Bed debris in the valley with the 

 pebbles transported from the basalt by the river to varying distances 

 below the termination of the flow, the greatest distance being 

 30 miles in the case of vesicular basalt. 



Although not of glacial origin itself, the fact that the Pebble-Bed 

 passes into true glacial deposits in the south cannot be doubted. 

 Among others Dr. Nordenskjceld has pointed this out, and gives as 



1 Anales Mus. Nac. Buenos Aires, ser. 2, vol. ii (1896-97) p. 113. 



2 Am. Journ. Sci. ser. 4, vol. iv (1897) p. 246. 



3 < Geol. Obs.' 2nd ed. (1876) p. 225. 



Q. J. G. S. No. 234. n 



