1854.] LYELL — GEOLOGY OF MADEIRA. 327 



or only at angles of 3° to 7°, whereas at a distance of two or three miles, 

 they become more tilted, being inclined at angles of 10°, 13°, and 

 even sometimes 20°. When steepest, they sometimes dip inwards, 

 but rarely, the inclination being usually away from the axis. Sir 

 Charles infers that all these lavas issued originally from vents, situated 

 in what is now the central and usually the highest part of Madeira. 

 They formed, together with the cones of eruption, a flattened dome, 

 having a slope varying from 3° to 7° or 8°=. They acquired their 

 present steeper inclination during the convulsions attending later 

 eruptions. He agrees, therefore, so far with the Erhebung theory 

 of Von Buch, as to admit an expansive movement operating with its 

 greatest intensity along the central axis ; but he supposes this move- 

 ment not to have been confined to one period. On the contrary, 

 Madeira affords evidence in several regions of great dislocation of the 

 rocks having been accomplished before other lavas and tufPs were in 

 existence. Thvis the Picos or Cones of Funchal and Camara de 

 Lobos, with their nearly horizontal lavas and tuffs, about 800 feet 

 thick, cover unconformably the older volcanic rocks of Cape Giram. 

 The newest member of these last consists of alternating lavas and 

 scoriae, which are tilted at an angle of 30° and 32°, and are overlaid 

 by the Funchal series just alluded to. 



Near Porto da Cruz also, a great series of slightly inclined trachytes 

 and tuffs are newer than the central cones of eruption with their 

 steeply-inclined basalts and tuffs, these last having been tilted, and 

 having had deep valleys eroded in them, before the trachytic outpour- 

 ings took place. Other facts are then adduced to prove that different 

 parts of Madeira have been formed in succession, and a suggestion is 

 thrown out, that the movements accompanying eruptions extend over 

 a much wider area than the superficial ejectamenta and lavas ; also 

 that the subterranean injection of fissures occurs over a wider space 

 below than the effects of eruptions above grornid. If so, each series 

 of lavas will most commonly be rent, tilted, and sometimes injected 

 during convulsions accompanying the formation of newer and neigh- 

 bouring vents and craters ; so that those lavas alone which proceed 

 from the most modern vents will remain wholly undisturbed so as to 

 retain their original position. 



Sir Charles remarks that the lavas of Madeira, whether vesicular 

 or compact, do not constitute continuous sheets parallel to each 

 other. When viewed in the sea-cliffs in sections transverse to the 

 direction in which they flowed, they vary greatly in thickness, even 

 if followed for a few hundred feet or yards, and they usually thin out 

 entirely in less than a quarter of a mile. In the ravines which radiate 

 from the centre of the island, the basaltic beds are more persistent, 

 but even here they usually are seen to terminate, if followed for a few 

 miles ; their thickness also being in some cases very variable. Occa- 

 sionally cones of scorise, made up of ejectamenta, inclined at 20° or 

 30°, are surrounded and buried by the lavas proceeding from the 

 central axis. 



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

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



VOL. X. PART I. 2 A 



