Vol.59'] GEOLOGY OF rATAGONIA. 167 



vicinity of the Cordilleras. This can be plainly discerned when 

 travelling from Lake Buenos Aires to the Chico de Santa Cruz, and 

 is so constant that one is not so much inclined to attribute it to 

 subsequent tectonic disturbances, as to regard it as the original angle 

 at which the lava solidified. It is even possible that the line along 

 which the angle changes is the line where the lava plunged into 

 the sea. 



Concerning the age of the basalt, nothing definite can be said in 

 this paper, as it was impossible to study for a sufficiently long time 

 the strata underlying it. M. Mercerat places it in the Upper 

 Tehuelche ; Darwin considered the flows on the Bio Santa Cruz to 

 be contemporaneous with the beds that have been called ' Upper 

 Patagonian ' by M. Mercerat. Judging from evidence that I have 

 seen, the outburst took place prior to the tectonic disturbances 

 which produced the great transverse valleys of the Cordilleras, and 

 before the glaciation of the eastern slopes of that range. The latter 

 point is proved by the abundance of basalt-erratics now lying on the 

 surface of the ground near the Cordilleras ; the former contention 

 is upheld by the fact that the disturbances which produced the 

 depression wherein Lake Buenos Aires lies have faulted down a 

 portion of the plateau on the southern shore, mentioned above, to the 

 level of the lake at the mouth of the Rio Fenix. 



V. The Tehuelche Pebble-Bed. 



This ' gravel-formation ' was first met with in the Colony of Chubut. 

 Thence to the Bio Mayo, either the bed itself, or pebbles in the 

 valleys derived from it, are always to be seen. Later, in the vicinity 

 of Santa Cruz, there were good opportunities of studying it. Before 

 describing the petrological characters of its constituents, I must 

 mention that at Port Madryn, in the Golfo Nuevo, which was 

 actually the first spot at which I saw the ' gravel-formation,' it is 

 probable that M. Mercerat would refer the deposit to one of the 

 older pebble-beds, since the matrix is arenaceous and contains an 

 Ostrea resembling ' 0. patagonicaj which it was unfortunately 

 impossible to preserve. 1 The bed in question occurs in a low cliff 

 on the beach, and underlying it is one of the mudstones mentioned 

 by Darwin. 2 Boiled fragments of the mudstone occur in the pebble- 

 bed, but it may be that this demonstrates nothing more than a local 

 check of short duration in the deposition, since at Monte Leon I 

 observed a similar phenomenon in the continuous series of false- 

 bedded grits and mudstones. If, then, this pebble-bed belongs to 

 the Upper Patagonian, it cannot be considered in connection with the 

 Tehuelche Pebble-Bed, except to note that it contains pebbles of 

 quartz-porphyry similar to those in the younger deposit. 



On ascending to the pampas above the valley of the Bio Chubut a 

 sheet of gravel is found to cover the surface. The thickness could 

 not be ascertained, but I am led to believe, by the alternate bare 



1 Mercerat, Anales Mus. Nac. Buenos Aires, ser. 2, vol. ii (1896-97) p 115 



2 ' Geol. Obs. : 2nd ed. (1876) p. 373. 



