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, aiid 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 oxydule (which is not distinguishable in the
basalt before decomposition) in black specks. I no where observed
this loose conglomerate in longitudinal lines, 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-
5 X 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 like 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 Teneriffe, by a Russian gentleman,
Mr. J. Thai. .