CHAP. VI. 



GEOLOGY. 135 



beds of -which they are formed were split at innumerable 

 points, and that the basaltic lava rose in every split, and 

 issued at every pore. It is further to be inferred, from what 

 has been said, that at a time anterior to this elevation of the 

 land a space equal to its area below the surface of the sea 

 had been covered with those beds of conglomerate, tufa, and 

 lava, which make up the body of the island. Nothing is 

 more probable than the supposition that the elevation itself 

 was due to the expansion and explosion of steam and gases, 

 and that the Curral and ravines of Madeira, as well as the 

 canadas and barrancos of Teneriffe, are due to the mechani- 

 cal effects of such an expansion riving the rocks, qua-qua- 

 versally, from a central crater of elevation. At Teneriffe the 

 phenomena are slightly modified by the repetition of erup- 

 tions in subsequent times ; but in Madeira there appears to 

 be no evidence of any eruptions not coeval with the island. 



As for the geological " locus" of the subterranean forces 

 of elevation and eruption, it appears to lie here, as in so 

 many other cases, in or beneath a trachytic formation. In 

 Teneriffe the crater of elevation is all trachytic. In Madeira 

 a well-characterized trachytic rock appears in the ravine of 

 Faial, near Porta da Cruz, beneath the crater of elevation, 

 which, as has been stated, consists entirely of basaltic tufas, 

 conglomerates, and lava. 



That there is a general identity in the source of all volcanic 

 action no one can doubt who observes the uniformity of vol- 

 canic products, under similar conditions, in all parts of the 

 world. It may be presumed, perhaps, that the correspond- 

 ence and contemporaneousness of earthquakes in distant 

 volcanic foci are due to this cause. One of the statements of 

 such a correspondence in Madeira is that " the undulation 

 from the Lisbon earthquake in 175,5 reached this island soon 



