130 GEOLOGY. [chap. vi. 



If we extend our geological view from these islands to the 

 Canaries, we do not find, at least in Teneriffe, the same fossil 

 bed ; but we perceive, nevertheless, in the mineral masses of 

 which that island is composed, evidence of its belonging to the 

 same formation. In Teneriffe the lofty central crest of moun- 

 tains, so well described by Von Buch as a crater of elevation, 

 consists of a trachytic rock, that is to say, a rock largely 

 felspathic, of a rough fracture, and marked by numerous 

 crystals of glassy felspar. Mantling round the base of the tra- 

 chytic mountains lie the basaltic strata, which line the coast, 

 and, associated with these, beds of cinders and tufa variously 

 coloured. The vineyards, both of Teneriffe and Madeira, are 

 planted in the tufas, several varieties of which readily form a 

 fruitful soil. Not only the general disposition of this series, 

 but many of the details of it, are identical in the two islands ; 

 and, in particular, the seams of tufa have the same peculiari- 

 ties of mineral contents. These beds are best seen along 

 a considerable section of the coast of Madeira, in the neigh- 

 bourhood of Funchal. They lie in horizontal strata, covering, 

 and interposed between, basaltic lavas, by which at many 

 points they are dislocated and pierced. Their stratification 

 is marked by the different colours of the seams— red, yellow, 

 brown, and black. Their substance seems to be no other 

 than that of disintegrated, incoherent, basalt, in different 

 states of oxygenation, as regards the iron with which they 

 are coloured. Where the red variety is consolidated in con- 

 tact with super-imposed streams of basaltic lava, it some- 

 times assumes, in both islands, the prismatic form of co- 

 lumnar basalt. Amongst the materials of which these strata 

 consist certain seams of pumice nodules, occupying the same 

 place in the yellow tufa of either island, is especially charac- 

 teristic. 



