170 MR. J. B. SCRIVENOR ON THE [May I903, 



his opinion that the bed was ' laid down by big rivers whose sources 

 were immense glaciers, and which, flowing through country with a 

 comparatively level surface, often altered their course.' He points 

 out that the bed cannot be said to cover the pampas entirely, but if 

 we assume the modern rivers to have approximately the same course 

 as their bigger predecessors, then it is possible to explain why sec- 

 tions of considerable thickness are found in the valleys. 1 



To me it seems that the origin of the Pebble-Bed must have been 

 complex. The glaciers which scoured the eastern slopes of the 

 Cordilleras have deposited at the heads of the Patagonian rivers a vast 

 amount of morainic material consisting of the rocks already described. 

 Porphyries are among them, but nowhere in sufficient quantities (in 

 my experience) to supply the requisite amount for the bed on the 

 eastern coast. On the melting of the snow and ice the torrents 

 would wear the fragments, and distribute them in fans over a con- 

 siderable area. Subsequent subaerial denudation might go far to 

 produce the appearance of a continuous and stratified deposit. At 

 this time the coast-line lay certainly farther west than at present ; 

 on that point there seems to be no difference of opinion. It 

 may even be that the coast-line had not yet receded far enough 

 eastward since the outpourings of basalt, to make the terminations of 

 those flows dry land. Supposing this to be the case, and admitting 

 that the torrents had brought the morainic material to the coast-line, 

 the sea would further distribute them, not only along its shores, but 

 over the bottom, as has been shown to be the case on the present 

 shore-line, 2 so that an uniform layer of pebbles would be deposited 

 over the basalt, which, as the sea receded more to the east, would 

 (if the supply of material was sufficient) extend over the limit of the 

 flows. This hypothesis will perhaps permit of the presence of the 

 hornblende-gneiss, granitic, and rhyolitic pebbles east of the basalt 

 without basalt-pebbles, but it must be frankly admitted that it is 

 not convincing. Yet the alternatives, that they have been derived 

 from masses in situ on the east of the basalt, or from older pebble- 

 beds deposited before the basaltic eruptions, draw even more on the 

 imagination, since no such masses as the former have been yet 

 discovered, and our knowledge of the distribution and contents of 

 the latter is insufficient. 



M. Mercerat, when speaking of the Pebble-Bed on the east of the 

 basalt, says that he follows Darwin's opinion, founded on observa- 

 tions at Port Desire, that the porphyry-pebbles have not been 

 derived from the masses found in that region in situ. 3 On the 

 other hand, Darwin admitted A that they may have come partly from 

 rocky ridges in the central districts of Patagonia. That masses of 

 quartz-porphyry, not necessarily forming ridges, do exist in greater 

 number than was known at the time of the voyage of the Beagle 

 is now certain, and the objection that the colours of the pebbles do 



1 Am. Geol. vol. xxi (1898) p. 303. 



2 » Geol. Obs.' 2nd ed. (1876) p. 214. 



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



4 Op. supra cit. p. 224. 



