AND PORTO SANTO. 23 



fracture, which becomes covered with a superficial, decomposing 

 coat of brownish red, with age, inducing brittleness, and passing 

 ultimately into yellowish white, and dull brown ; a moderately thin 

 plate of the columnar basalt, when covered exteriorly with red 

 powder, will snap between the fingers. There are narrow beds 

 both above and below these columns, and frequently between 

 them, of shapeless fragments of basalt, very frangible, of an earthy 

 fracture, generally about the size of a walnut, imbedded in, or 

 thickly coated with a friable earth, resembling the tufa. In the 

 columnar basalt immediately above the beach, this conglomerate 

 is not above eight inches deep, the imbedding earth resembles the 

 yellow tufa, and the fragments are not porous, which they are, 

 minutely, throughout the inland section, about a quarter of a mile 

 behind it, on the right as you descend to the second ravine west 

 of the town. Descending this ravine to the beach, we have 

 columns of porous basalt on the right, with beds of conglomerate 

 above and below it, the latter about four feet, and the former about 

 six in its greatest depth : the fragments imbedded are here much 

 larger, in some instances have lost their colouring matter entirely, 

 and disclose the fer oxr/dule (which is not distinguishable in the 

 basalt before decomposition) in black specks. I no where ob- 

 served this loose conglomerate in longitudinal hnes, or patches 

 between the columns, (which were always vertical) but only in 

 beds above and below them, and that, not only in the direction of 

 the dip, but in that of the drift line; wherefore I concluded, that 

 it could not be the result of a partial decomposition of the basalt, 

 and this was afterwards confirmed, by finding large masses of lava, 

 as perfect as that from the crater of TenerifFe', imbedded con- 



' I have no memoranda either of Haiiy's or Faujas' classification of lavas, but this 

 is of a reddish brown, and might well be called vermiform, for it looks hke a surface 

 covered with a mass of leeches, erecting their bodies as if in the act of regorging. I 

 compared it with a specimen just brought from TeneriflPe, by a Russian gentleman, 

 Mr. J. Thai. 



