Geology i and CHmate of the Island of Madeira. 349 



are composed of the same materials with some of the igneous 

 rocks, but which, in a comminuted state, have been borne 

 down during volcanic eruptions by the torrents of water pro- 

 duced by the condensation of the vast volumes of vapour then 

 evolved. The Cape is named by the English "• Brazen Head," 

 from the red colour of the strata, which in that situation being 

 much disturbed, and intersected by dark basaltic dykes, pre- 

 sent from the sea a very peculiar appearance. Immediately to 

 the west of the Pontinha, a similar aqueous deposit may be ob- 

 served along the chff. In this locality it is overcapped by a 

 thick bed of compact basalt, which has been poured out over 

 it. The heat of this superincumbent mass has given rise to an 

 interesting appearance. The upper part of the layer of the 

 red aqueous stratum, which has been in contact with the ba- 

 salt, has been altered by the intense heat, and, in cooling, has 

 assumed the columnar structure. A very fine effect is thereby 

 produced, the dark amorphous mass of basalt being supported 

 by lines of small red pillars of an angular form. This is seen 

 for a considerable distance along the coast, but the appearance 

 is especially beautiful at the mouth of the small stream the 

 Ribeiro Seco, where a large extent of this columnar formation 

 is exposed." 



Aerial Volcanic Rocks. — Between the points just mentioned, 

 the Pontinha and the Ribeiro Seco, may be also observed a 

 series of aerial volcanic products. They consist of pumice, 

 sand, and the various matters ejected by the expansive force 

 of vapours or gases. The different layers, some of which are 



* Mr Bowditch (Excursions, p. 24) remarks, that the red tufa, which has 

 undergone this change of form, is increased in specific gravity from 1.75 to 

 2.0G. I am not aware whether this effect of volcanic streams, in thus de- 

 termining a columnar structure in previously deposited rocks, is of frequent 

 occurrence, or whether it has been described in reference to any other loca- 

 lity. Mr Smith informs me, that near Glasgow a whin dyke passing througli 

 a coal bed has deprived the coal of its bitumen, and rendered it columnar. 

 Since being in Madeira, I have observed (Juno 1840) a line appeai'ance of the 

 same kind on the south side of the hill of Gcrgovia, ne:vr Clermont in Au- 

 voi-gne, where the basalt has flowed over limestone, and caused it to assume, 

 in like manner, the columnar form, the little pillars for some distance pro- 

 nenting considerable regularity. 



VOL. XXIX NO, LVm. OCTOBER 1840. A a 



