



So Tertiary Formations. part n. 



the Atlantic and the Cordillera, we have the preceding 

 section. 



The upper half of the sedimentary mass, under the 

 basaltic lava, consists of innumerable zones of perfectly 

 white, bright green, yellowish and brownish, fine-grained, 

 sometimes incoherent, sedimentary matter. The white, 

 pumiceous, trachytic tuff-like varieties are of rather 

 greater specific gravity than the pumiceous mudstone 

 on the coast to the north : some of the layers, especially 

 the browner ones, are coarser, so that the broken crys- 

 tals are distinguishable with a weak lens. The layers 

 vary in character in short distances. With the excep- 

 tion of a few of the Osbrea Patagonia, which appeared 

 to have rolled clown from the cliff above, no organic 

 remains were found. The chief difference between 

 these layers taken as a whole, and the upper beds both 

 at the mouth of the river and on the coast northward, 

 seems to lie in the occasional presence of more colouring 

 matter, and in the supply having been intermittent : 

 these characters, as we have seen, very gradually dis- 

 appear in descending the valley, and this fact may 

 perhaps be accounted for by the currents of a more 

 open sea having blended together the sediment from a 

 distant and intermittent source. 



The coloured lavers in the foregoing: section rest on 

 a mass, apparently of great thickness (but much hidden 

 by the talus), of soft sandstone, almost composed of 

 minute pebbles, from one-tenth to two-tenths of an 

 inch in diameter, of the rocks (with the entire exception 



a great slip, which has formed hills between 60 and 70 feet in height, 

 and has tilted the strata into highly inclined and even vertical posi- 

 tions. The strata generally dipped at an angle of 15° towards the 

 cliff from which they had slided. I have observed in slips, both on 

 a small and large scale, that this inward dip is very general. Is it 

 due to the hydrostatic pressure of water percolating with difficulty 

 through the strata acting with greater force at the base of the 

 mass than against the upper part ? 



