Vol.59-] GEOLOGY OF PATAGONIA. 171 



not exactly match those of the masses near which they are found 

 cannot he very serious, seeing how the pebbles may have been 

 washed along the old shore-line and then again disturbed by the 

 present rivers. That the porphyry-pebbles have been in part derived 

 from the Cordilleras is quite probable ; they occur on the southern 

 and northern shores of Lake Buenos Aires, in the bed of the Rio 

 Fenix, in that of the Rio de los Antiguos, on the floors of the valleys 

 from the Fenix to the Belgrano — indeed everywhere I saw them, 

 but never, under the Cordilleras, bearing such a ratio to the other 

 granitic, gneissose, and rhyolitic pebbles as to admit of those 

 mountain-chains being the sole source from which the pebbles in the 

 bed on the eastern coast were drawn. There really seems to be no 

 valid reason why the portion of the bed east of the basalt should not 

 nave been laid down by the action of a sea gradually receding, but 

 with long enough breaks for the formation of the steps which lead 

 from one pampa to another as they rise to the Cordilleras, the 

 material being chiefly derived from islets of quartz-porphyry. 



It may be objected that, since the pebble-bed (mentioned on p. 167) 

 at Port Madryn contains the same pebbles as the Tehueiche Pebble- 

 Bed, and yet occurs in such close association with strata which have 

 elsewhere (at Port St. Helena) been metamorphosed by the intrusive 

 mass of quartz-porphyry, it is improbable that the material could 

 nave been derived from that mass or any mass of the same date of 

 irruption. To this there are two answers : — Firstly, the relative age 

 of the mudstones at Port Madryn and Port St. Helena is not clear : 

 those at the latter locality may quite possibly be older and owe their 

 position to disturbances of the crust ; secondly, seeing that in the 

 valley of the Chico de Chubut a foliated and faulted mass occurs 

 not far from a mass of similar composition, but showing no signs of 

 stress, it is impossible that the igneous masses of the Chubut terri- 

 tory were irrupted at the same time. 



Can the Tehueiche Pebble-Bed have had a purely fluviatile origin 

 after the requisite material had once been detached from the 

 Cordilleras by the glaciers ? This would necessitate the transporta- 

 tion of the gneissose and granitic rocks found at Santa Cruz and 

 Monte Leon from the Cordilleras by river-power only. Then it 

 follows that numbers of basalt-pebbles would be brought with them, 

 whereas they do not exist. Again, Darwin cites, as an instance of 

 the insignificant transporting power of the Rio Santa Cruz, which has 

 a speed of 5 knots, the fact that dense basalt could not be found in its 

 bed at a greater distance than 10 miles from the end of the flows, 

 nor vesicular at a greater distance than 30. l How, then, could the 

 granitic and gneissose pebbles be transported from the moraines ? 

 If a stronger stream is brought into being to do the work, it affords 

 also a stronger argument for expecting basalt-pebbles at the coast. 



M. Mercerat says that the pebbles increase in size towards 

 the Cordilleras.* This is a generalization to which I would not 

 commit myself, unless I had seen a great deal more of the 



1 < Geol. Obs.' 2nd ed. (1876) p. 224. 



2 Anales Mus. Nac. Buenos Aires, ser. 2, vol. ii (1896-97) p. 114. 



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