374 Tertiary Formations. paet n. 



the harbour. The shells sufficiently perfect to be 

 recognised consist of: — 



1. Ostrea Patagonica, d'Orbig. (also at St. Fe and whole coast of 



Patagonia). 



2. Pecten Paranensis, d'Orbig. 'Toy. Pal.,' and PI. III. f. 30 of this 



work (also at St. Fe, S. Josef. S. Julian). 



3. Pecten centralis, G. B. Sowerby, PL III. f. 31 (also S. Julian 



and S. Cruz). 



4. Cuculleea alta, do., PL II. f. 22, 23 (also S. Cruz). 



5. Xucula ornata, do., PL II. f. 19. 



6. Turritella Patagonica, do., PL III. f. 48. 



The fossiliferous strata, where not denuded, are 

 conformably covered by a considerable thickness of the 

 fine-grained puniiceous mudstone. divided into two 

 masses : the lower half is very fine-grained, slightly 

 unctuous, and so compact as to break with a semi- 

 conchoidal fracture, though yielding to the nail ; it 

 includes laminas of selenite : the upper half precisely 

 resembles the one layer at the Kio Negro, and, with the 

 exception of being whiter, the upper beds at San Josef 

 and Xuevo Gulf. In neither mass is there any trace to 

 the naked eye of organic forms. Taking the entire 

 deposit, it is generally quite white, or yellowish, or 

 feebly tinted with green; it is either almost friable 

 under the finger, or as hard as chalk ; it is of easy 

 fusibility, of little specific gravity, is not harsh to the 

 touch, adheres to the tongue, and when breathed on 

 exhales a strong aluminous odour ; it sometimes con- 

 tains a very little calcareous matter, and traces (besides 

 the included laminae) of gypsum. Under the microscope, 

 according to Prof. Ehrenberg, 1 it consists of minute, 

 triturated, cellular, glassy fragments of pumice, with 

 some broken crystals. In the minute glassy fragments, 

 Prof. Ehrenberg recognises organic structures, which 

 have been affected by volcanic heat : in the specimens 

 from this place, and from Port S. Julian, he finds six- 



1 * Monatsberichten der konig. Akad. zu Berlin vom April, 1815.' 



