5 6 4 Valley of Copiapo. part n. 



300 feet in thickness of red sandstone ; and above this, 

 a stratum of black calcareous slate-rock, like that 

 which alternates with and replaces the underlying 

 yellowish white, siliceous sandstone. Close to the 

 junction between this upper black slate-rock and the 

 upper red sandstone, I found the Gryphcea Darwinii, 

 the Turritella Andii, and vast numbers of a bivalve, 

 too imperfect to be recognised. Hence we see that, as 

 far as the evidence of these two shells serves — and the 

 Turritella is an eminently characteristic species — the 

 whole thickness of this vast pile of strata belongs to 

 the same age. Again, above the last-mentioned upper 

 red sandstone, there were several alternations of the 

 black, calcareous slate-rock ; but I was unable to ascend 

 to them. All these uppermost strata, like the lower ones, 

 vary extremely in character in short horizontal distances. 

 The gypseous formation, as here seen, has a coarser, 

 more mechanical texture, and contains much more 

 siliceous matter than the corresponding beds lower 

 down the valley. Its total thickness, together with the 

 upper beds of the porphyritic conglomerate, I estimated 

 at least at 8,000 feet ; and only a small portion of the 

 porphyritic conglomerate, which on the eastern flank of 

 the fourth axis of elevation appeared to be from 1,500 

 to 2,000 feet thick, is here included As corroborative 

 of the great thickness of the gypseous formation, I 

 may mention that in the Despoblado Valley (which 

 branches from the main valley a little above the town 

 of Copiapo) I found a corresponding pile of red and 

 white sandstones, and of dark, calcareous, semi-jaspery 

 mudstones, rising from a nearly level surface and 

 thrown into an absolutely vertical position ; so that, by 

 pacing, I ascertained their thickness to be nearly 2,700 

 feet ; taking this as a standard of comparison, I esti- 



