652 TRACHYTIC ROCKS. [Ch. XXIX. 



In the Alps, no doubt, as in other lofty chains, the formation of 

 valleys has been greatly aided by subterranean movements, both 

 gradual and violent, and by the dislocation of rocks. The same may 

 be true of Madeira and of almost every lofty volcanic region ; but, 

 when we reflect that the central heights a and b, fig. 706, are more 

 than 6000 feet above the sea, and that the waters flowing from them, 

 swollen by melted snows, reach the sea by a course of not much 

 more than 6 miles in the case of those draining the Curral, and by 

 nearly as short a route in the Serra d'Agoa, we shall be prepared 

 for almost any amount of denudation effected simply by subaerial 

 erosion. 



The general absence of water-worn pebbles in the tuffs underlying 

 the Madeira lavas is very striking, and contrasts with the frequent 

 occurrence of gravel-beds under so many of the Auvergne lavas. It 

 simply proves that Madeira, like the volcanic mountains of Java, or 

 like Mount Etna or Mona Loa in the Sandwich Islands, could not, 

 so long as eruptions were frequent, and while the porous lavas ab- 

 sorbed all the rain-water, support a single torrent on its slopes.* 

 The period, therefore, of fluviatile erosion must have been almost 

 entirely subsequent in date to the formation of the central nucleus 

 of ejectamenta, c, fig., p. 648, and of the lavas d, ibid. When we 

 infer that these were of supramarine origin as far down as the line 

 p, s, t, and perhaps lower, it follows that a lofty island, 4000 feet or 

 more in height, must have resulted, even if no upheaval had ever 

 occurred. 



The movements which upraised the marine deposits of San Vicente 

 may or may not have extended over a wide area. How far, when 

 they occurred, they modified the form of the island, or added to its 

 height, is a fair subject of speculation ; and whether the steep dip of 

 the lavas seen in the ravines intersecting the slopes of the mountain, 

 / h and e g (fig. f 06, p. 648), may be ascribable in part to such 

 movements. The lavas of more modern date, near Funchal, may be 

 imagined to remain comparatively horizontal, because they have 

 escaped the influence of disturbing forces to which the older nucleus 

 was exposed. Without discussing this point (so fully treated of in 

 reference to Palma), I may observe that unquestionably different 

 parts of Madeira have been formed in succession. Near Porto da 

 Cruz, for example, on the northern coast, trachytes of a gray and yel- 

 low, and trachytic tuffs almost of a white color, in slightly inclined 

 or almost horizontal beds, have partially filled up deep valleys previ- 

 ously excavated through the older and inclined basaltic rocks (dip- 

 ping at an angle of 10° to the north), under which the leaf-bed and 

 lignite before mentioned (fig. 706, p. 648) lie buried. During the 

 convulsions which accompanied the outpouring of every newer series 



* See remarks on Etna, Ly ell's Principles of Geology, chapter xxv. (9th e&, 

 p. 405). 



