512 ISLAND OF MADEIRA. [Cii. XX IX. 



and referable perhaps to the Miocene tertiaiy epoch. Tuffs and hoie- 

 stones containing marine shells and corals occur at S. Vicente on the 

 northern coast, where they rise to the height of more than 1200 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 ag- 

 glomerates formed above the level of the sea. 



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

 fi'om north to south being 12 miles. The annexed section, fig. 653, 

 drawn up on a true scale of heights and horizontal distances from the 

 observations of Mr. Hartung and myself, will enable the reader to com- 

 prehend 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 accumu- 

 lation 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 

 scoria, 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 of the 

 volcanic mass, below^ f h and e </, consists almost exclusively of streams 

 or sheets of basalt, with some red partings of ochreous clay or laterite, 

 probably ancient soils. The darker lines indicate the predominance 

 of these lavas wdiich have flowed on the surface, and which, besides 

 basalt, consist of various kinds of trap, and in some places of trachyte. 

 The lighter tint, c, expresses an accumulation of scoriae, agglomerate, 

 and other materials, such as may have been piled up in the open 

 air, in or around the chief orifices of eruption, and between volcanic 

 cones. 



The Pico Torres, a, more than 6000 feet high, is one of many central 

 peaks, composed of ejected materials. By the union of the foundations 

 of many similar peaks, ridges or mountain crests are formed, from which 

 the tops of vertical dikes project like turrets above the weathered surface 

 of the softer beds of tuff' and scoria2. Hence the broken and picturesque 

 outline, giving a singular and romantic character to the scenery of the 

 highest part of Madeira. North of a is seen Pico Ruivo (b), the most 

 elevated peak in the island, yet exceeding by a few feet only the height 

 of Pico Torres. It is similar in composition, but its uppermost part, 

 400 feet high, retains a more perfectly conical form, and has a dike at 

 its summit with streams of scoriaceous lava adhering to its steep flanks. 

 There are a great many such peaks east and west of a, which seem to be 

 the ruins of cones of eruption, the materials of some at least having 

 been arranged with a qua-qua-versal dip. Among these is Pico Grande, 

 c, fig. 655, now half-buiied under more modern lavas which have 

 flowed round it. It is perhaps owing to the juxtaposition of such a 

 multitude of cones or points of eruption, and the interference of their 



