586 Summary of the Structure paetil 



into green-stone, was occasionally deposited between 

 the beds of erupted matter : with this exception the 

 uniformity of the porphyritic rocks is very remarkable. 



At the period when the clay-stone and green-stone 

 porphyries nearly or quite ceased being erupted, that 

 great pile of strata which, from often abounding with 

 gypsum, I have generally called the Gypseous formation 

 was deposited, and feldspathic lavas, together with 

 other singular volcanic rocks, were occasionally poured 

 forth : I am far from pretending that any distinct line 

 of demarkation can be drawn between this formation 

 and the underlying porphyries and porphyritic con- 

 glomerate, but in a mass of such great thickness, and 

 between beds of such widely different mineralogical 

 nature, some division was necessary. At about the 

 commencement of the gypseous period, the bottom of 

 the sea here seems first to have been peopled by shells, 

 not many in kind, but abounding in individuals. At 

 the P. del Inca the fossils are embedded near the base 

 of the formation ; in the Peuquenes range, at different 

 levels, half-way up, and even higher in the series; 

 hence, in these sections, the whole great pile of strata 

 belongs to the same period : the same remark is appli- 

 cable to the beds at Copiapo, which attain a thickness of 

 between 7,000 and 8,000 feet. The fossil shells in the 

 Cordillera of Central Chile, in the opinion of all the 

 palaeontologists who have examined them, belong to 

 the earlier stages of the cretaceous system ; whilst in 

 Northern Chile there is a most singular mixture of cre- 

 taceous and oolitic forms : from the geological relations, 

 however, of these two districts, I cannot but think that 

 they all belong to nearly the same epoch, which I have 

 provisionally called cretaceo-oolitic. 



The strata in this formation, composed of black 

 calcareous shaly-rocks of red and white, and sometimes 



