Ch. XXIX.] TRACHYTIC EOCKS. 517 



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, for reasons before given, 

 p. 475, support a single torrent so long as eruptions were frequent on its 

 slopes. The period, therefore, of fluviatile erosion must have been sub- 

 sequent to the formation of the central nucleus of ejectameuta, c, fig., 

 p. 513, and of the lavas, c?, ibid. When we infer that these were of 

 supramarine origin as far down as the hne p, , ^, 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 they modified 

 the form of the island, or added to its height, is a fair subject of specula- 

 tion ; and whether the steep dip of the lavas seen in the ravines inter- 

 secting the slopes of the mountain, / h and e </, can be ascribed \ o such 

 movem.ents. 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 trachytic tuffs almost of a white color, 

 in slightly inelined or almost horizontal beds, have partially filled up 

 deep valleys previously excavated through the older and inclined basaltic 

 rocks (dipping at an angle of 10° to the north), under which the leaf-bed 

 and lignite before mentioned, fig. 653, p. 513, lie buried. During the 

 convulsions which accompanied the outpouring of every newer series of 

 lavas, the older rocks may have been more or less disturbed and tilted, 

 without destroying the general form of the- old dome-shaped mountain 

 supposed by us to have been the result of repeated eruptions from the 

 central vents. 



The locality just referred to of Porto da Cruz exemplifies, not only 

 the long intervals of time which separated the outflowing of distinct sets 

 of lavas, but also the precedence of the basaltic to the trachytic out- 

 pourings. So also on the southern slope of Madeira, I observed between 

 the Jardim and Pico Bodes, situated in a direct line about six miles north- 

 west of Funchal, a well-marked series of trachytic rocks of considerable 

 thickness occupying the highest geological position. They consist of 

 white and gray trachytes, occuri'ing at points varying from 2500 to 3500 

 feet above the sea. Their position may be understood by supposing 

 them to constitute the uppermost beds represented at h in the section, 

 fig. 653, p. 513, and on the slope above h. The doctrine, therefore, that 

 in each series of volcanic eruptions the trachytic lavas flow out first, and 

 after them the basaltic kinds (see p. 522), is by no means borne out ia 



