chap. xv. Conglomerate with Silicified Wood. 565 



mated the thickness of the strata ahove the porphyritic 

 conglomerate at 7,000 feet. 



The fossils before enumerated, from the limestone- 

 layers in the whitish siliceous sandstone, are now 

 covered, on the least computation, by strata from 5,000 

 to 6,000 feet in thickness. Professor E. Forbes thinks 

 that these shells probably lived at a depth of from about 

 thirty to forty fathoms, that is from 180 to 240 feet; 

 anyhow, it is impossible that they could have lived at 

 the depth of from 5,000 to 6,000 feet. Hence in this 

 case, as in that of the Puente del Inca, we may safely 

 conclude that the bottom of the sea on which the shells 

 lived, subsided, so as to receive the superincumbent 

 submarine strata : and this subsidence must have taken 

 place during the existence of these shells ; for, as I 

 have shown, some of them occur high up as well as low 

 down in the series. That the bottom of the sea sub- 

 sided, is in harmony with the presence of the layers of 

 coarse well-rounded pebbles included throughout this 

 whole pile of strata, as well as of the great upper mass 

 of conglomerate from 2,000 to 3,000 feet thick ; for 

 coarse gravel could hardly have been formed or spread 

 out at the profound depths indicated by the thickness 

 of the strata. The subsidence, also, must have been 

 slow to have allowed of this often-recurrent spreading 

 out of the pebbles. Moreover, we shall presently see 

 that the surfaces of some of the streams of porphyritic 

 lava beneath the gypseous formation, are so highly 

 amygdaloidal that it is scarcely possible to believe that 

 they flowed under the vast pressure of a deep ocean. 

 The conclusion of a great subsidence during the exist- 

 ence of these cretaceo-oolitic fossils, may, I believe, 

 be extended to the district of Coquimbo, although 

 owing to the fossiliferous beds there not being directly 



