22 EXCURSIONS IN MADEIRA 



most symmetrical prisms are exposed on the faces of the moun- 

 tains, and form the most elevated range at the sides of the inland 

 valleys, where they could never have been in contact with the 

 torrents which flow beneath, and which could not have existed 

 when the streams of basalt were first ejected from the crater. 

 The opinion, that the columnar structure natural to the basalt has 

 been developed by the continued action of streams of water, which 

 formerly washed, but now flow beneath it, from having in the 

 course of ages hollowed out a much deeper bed than that which 

 it occupied in the first instance; this opinion struck me as very 

 probable, until I remarked the position of the columnar basalt just 

 alluded to, and represented in the coloured sketches, Plate 3, A, C, 

 and of that exposed high up on the sides of mountains, remote 

 from valleys, where it must have always been, Hke the columnar 

 basalt crowning the cUflFs at the sea-side, out of the reach of the 

 action of water, unless we take into account the heavy rains, and 

 the torrents poured out from the crater during eruptions, causes, 

 seemingly too transient, to be adequate to the effect, which could 

 only be ascribed to a long and continued action on the part of the 

 waters. Could the above opinion have been established, we might 

 have estimated, by comparing the actual depth of these ravines, or 

 valleys, with the observed increase of that depth, in a known period 

 of time, the probable age of the basalt. The valleys or ravines of 

 Madeira have, of course, been deepened by the agency of water, but 

 I cannot consider that they have been entirely formed by it : the 

 various and partial directions of the streams of basalt, as they de- 

 scended from the crater to the sea, and the pre-existing hills and 

 valleys (for we shall presently discover that the base of the island 

 is of a transition, if not of a primitive formation), being no doubt 

 the primary causes. 



The columnar basalt is, generally speaking, compact, of a broad 

 conchoidal fracture, splitting in horizontal laminae, or at right 

 angles to the prism, dark grey within, or on the surface of a fresh 



