90 EXrTTRSrOM<3 IN MADEIRA 



tufa of Porto Santo, but through the conglomerates, limestones, and 

 tufa of Baxo, is probably coeval with the forcible elevation of these 

 islands ; the fissures being created by the convulsions preceding it. 

 One thing, however, seems certain, that the sandstone at Porto 

 Santo, which is not intersected by these dikes, was deposited sub- 

 sequently to the appearance of the shelly limestone. Whether it 

 was hove up from beneath, as I am much inclined to beheve, or 

 whether it became visible above the waters from a depression of 

 the channel of the ocean, I can say nothing in favour of the 

 hypothesis, that the heights of Madeira, Porto Santo and the 

 Canaries, may have formerly made part of a chain of primitive 

 mountains, distinct from, or continuing, the present western ex- 

 tremity of the Atlas. The limestone beneath the basalt at 

 Madeira, is evidently of the same nature and formation as that 

 beneath the basalt at Lisbon. The shelly limestone of Baxo is 

 distinct from that at Almada on the TagusP, but it is probably of 

 the same formation as the shelly limestone, mentioned by Baron 

 de Humboldt, as covered by basalt on the coasts of Portugal. Of 

 what formation is the limestone found on the coast of Africa, op- 

 posite to Teneriffe? and does that, subordinate to the tufa at 

 Lancerota and Forteventura, resemble either of those at Baxo^? 



The deposit of the sand stone on the plain of Porto Santo, seems 

 to have been providential, for it has enabled the inhabitants 

 (about 1400) to cultivate the vine, which would not succeed in 

 the calcareous clayey tufa, which yields them good crops of wheat, 

 Indian corn (zea-mays), barley', beans, and peas ; forming a con- 

 trast of cultivated vegetation, particularly striking in a small island, 



P Vide supra. 



1 Humboldt, Rel. Hist. I., 1., c- ii., and Supplement p. (541. 4to. 



' The produce of Porto Santo, in 1813, was 695 pipes of wine, 3768 bushels of 

 wheat, and 1628 bushels of barley. The population amounts to 1400, and there are 

 300 militia. M. Laplace, in Paris, and Mr. Morton Pitt, in a village of Devonshire, 

 found, that the number of men capable of bearing arms, amounted to J-th of tlie whole 



