AND PORTO CANTO fiO 



Porto Santo ; but the calcareous tufa just described, which is a 

 tertiary formation", seems to form its base, or at all events, is 

 lower than the present level of the sea, in the whole circumference 

 of the island. If we assume, that there are transition, or secon- 

 dary rocks beneath tliis tufa, and hidden by the sea, we must also 

 assume, in order to admit the pre-existence of Porto Santo as a 

 secondary formation, like Madeira, that the sea has been at a lower 

 level than it is now, or, that the shock which rent the previous 

 formation to admit of the throwing up of the clayey tufa, also 

 undermined these older rocks, and buried every vestige of the 

 former island beneath the ocean. When we associate the circum- 

 stance of the shelly hmestone, in the adjacent island of Baxo, being 

 160 feet above the sea, and about sixty feet above a similar deposit 

 of tufa, it seems to be much more natural to conclude, that both 

 Porto Santo and Baxo were formed beneath the ocean, and after- 

 wards hove vip, at a comparatively late period. Viewing Porto 

 Santo apart, it would be more simple to conclude, that the tufa, 

 (which is deposited confusedly, and not in beds) created by a sub- 

 marine volcano, was added, heap upon heap, and thus became 

 raised above the water ; in which case, (recollecting that it is found 

 at a height of 1600 feet above the sea, in the north-eastern parts 

 of the island) we must have admitted, that it continued to flow 

 through some crater or opening, long after the first emerging of 

 the island. But this reasoning is not applicable to the adjacent 

 island, which presents the shelly hmestone above the tufa, and 

 which has evidently been separated from Porto Santo. The basalt 

 which caps the peaks, and descends in dikes, not only through the 



° Combined with the limestone and sandstone, it seems to resemble the local 

 marney formation described by D'Aubuisson, at the foot of the Pyrenees, more than 

 any other, {Traite de Geognosie, vol. II. p. 436) ; miless it be the argile muriatijere, 

 sandstone, and shelly limestone brecciae at Araya, near Cumana. — Humboldt, Relation 

 Historique, 1. 2, c. v. 



N 



