Q4:6 ISLAND OF MADEIRA. [Ch. XXIX. 



might flow first from e to c, and then from a to c, fig. 705, so that the 

 slope might begin to resemble that formed by lavas and ejectamenta 

 from the summit a to Guia, on the southwestern side of the cone. 



Madeira. — Every volcanic island, so far as I have examined them, 

 varies from every other one in the details of its geographical and 

 geological structure so greatly that I have no expectation of finding 

 any simple hypothesis, like that of " elevation craters," applicable to 

 all, or capable of explaining their origin and mode of growth. Few 

 islands, for example, resemble each other more than Madeira and 

 Palma, inasmuch as both consist mainly of basaltic rocks of subaerial 

 origin, but, when we compare them closely together, there is no end 

 to the points in which they differ. 



The oldest formation known in Madeira is of submarine volcanic 

 origin, and referable to the Upper Miocene tertiary epoch, as will be 

 explained in Chap. XXXI., p. 665. To this formation belong the tuffs 

 and limestones containing marine shells and corals which occur at S. 

 Vicente on the northern coast, where they rise to the height of more 

 than 1300 feet above the sea. They bear testimony to an upheaval to 

 that amount, at least since the commencement of volcanic action in 

 those parts. 



The pebbles in these marine beds are well rounded and polished, 

 strongly contrasting in that respect with the angular fragments of 

 similar varieties of volcanic rocks so frequent in the superimposed tuffs 

 and agglomerates formed above the level of the sea. 



The length of Madeira from east to west is about 30 miles, its 

 greatest breadth from north to south being 12 miles. The annexed 

 section, fig. 705, drawn upon a true scale of heights and horizontal 

 distances from the observations of M. Hartung and myself, will enable 

 the reader to comprehend some of the points in which, geologically 

 considered, Madeira resembles or varies from Palma, In the central 

 region, at a, as well as in the adjoining region on each side of it, are 

 seen, as in the centre of Palma, a great number of dikes penetrating 

 through a vast accumulation of ejectamenta, c. Here also, as in 

 Palma, we observe as we recede from the centre, that the dikes 

 decrease in number, and beds of scorise, lapilli, agglomerate, and tuff 

 begin to alternate with stony lavas, d, d, until at the distance of a mile 

 or more from the central axis the volcanic mass, below/, h, and e, g, 

 consists almost exclusively of streams or sheets of basalt, with many 

 red partings of laterite or red ochreous clay. These red beds vary in 

 thickness from a few inches to two or three feet, and conskt sometimes 

 of layers of tuff, sometimes of ancient soils derived from decomposed 

 lava, both of them burnt to a brick-red color, and altered by the contact 

 of melted matter which has flowed over them. Some of these bands 



are represented in fig. 706, by interrupted lines, . The darker 



divisions with vertical cross-bars |iil!ill!ii!lilS!ll indicate lavas which originally 

 flowed on the surface. Had there been room, many more alternations 

 of such lavas would have been introduced. They consist chieflv of 



