588 Summary of the Structure paet n, 



characters and external features, might be compared to 

 the Silurian regions of North Wales. The gypseous 

 formation, like that of the porphyritic breccia-conglo- 

 merate on which it rests, is of inconsiderable breadth ; 

 though of greater breadth in Northern than in Central 

 Chile. 



As the fossil shells in this formation are covered, in 

 the Peuquenes ridge by a great thickness of strata, at 

 the Puente del Inca by at least 5,000 feet ; at Coquimbo, 

 though the superposition there is less plainly seen, by 

 about 6,000 feet; and at Copiapo certainly by 5,000 

 or 6,000, and probably by 7,000 feet (the same species 

 there recurring in the upper and lower parts of the 

 series) we may feel confident that the bottom of the 

 sea subsided during this cretaceo-oolitic period, so as 

 to allow of the accumulation of the superincumbent 

 submarine strata. This conclusion is confirmed by, or 

 perhaps rather explains, the presence of the many beds 

 at many levels of coarse conglomerate, the well-rounded 

 pebbles in which we cannot believe were transported in 

 very deep water. Even the underlying porphyries at 

 Copiapo, with their highly amygdaloidal surfaces, do 

 not appear to have flowed under great pressure. The 

 great sinking movement thus plainly indicated, must 

 have extended in a north and south line for at least 

 400 miles, and probably was co-extensive with the 

 gypseous formation. 



The beds of conglomerate just referred to, and the 

 extraordinarily numerous silicified trunks of fir-trees 

 at Los Hornos, perhaps at Coquimbo and at two dis- 

 tant points in the valley of Copiapo, indicate that land 

 existed at this period in the neighbourhood. This land, 

 or islands, in the northern part of the district of 

 Cop'apo, must have been almost exclusively composed, 

 judging from the nature of the pebbles, of granite ; in 



