532 
ELEMENTS OF GEOLOGY. 
Fig. 602. 
Fij?. 603. 
Ground-plan of dikes near Palagonia. 
a. Lava. b. Peperino, consisting of volcanic sand, or, in Other 
mixed wjth fragments of lava and limestone. 
of the Mediterranean 
where the waves have 
recently washed away 
the new volcanic isl¬ 
and ; for when a super¬ 
incumbent mass of 
ejected fragments has 
been removed by de¬ 
nudation, we may ex¬ 
pect to see sections of 
dikes traversing; tuff, 
words, sec¬ 
tions of the channels of 
communication by which the subterranean lavas reached the 
surface. 
Madeira ,—Although the more ancient portion of the vol¬ 
canic eruptions by which the island of Madeira and the 
neighboring one of Porto Santo were built up occurred, as 
we shall presently see, in the Upper Miocene Period, a still 
larger part of the island is of Pliocene date. That the latest 
outbreaks belonged to the Newer Pliocene Period, I infer 
from the close affinity to the present flora of Madeira of the 
fossil plants preserved in a leaf-bed in the north-eastern part 
of the island. These fossils, associated with some lignite in 
the ravine of the river San Jorge, can none of them be proved 
to be of extinct species, but their antiquity may be inferred 
from the following considerations: Firstly — The leaf-bed, 
discovered by Mr. Hartung and myself in 1853, at the height 
of 1000 feet above the level of the sea, crops out at the base 
of a cliff formed by the erosion of a gorge cut through alter¬ 
nating layers of basalt and scoriae, the product of a vast suc¬ 
cession of eruptions of unknown date, piled up to a thickness 
of 1000 feet, and which were all poured out after the plants, 
of which about twenty species have been recognized, flour¬ 
ished in Madeira. These lavas are inclined at an angle of 
about 15° to the north, and came down from the great cen¬ 
tral region of eruption. Their accumulation implies a long 
period of intermittent volcanic action, subsequently to which 
the ravine of San Jorge was hollowed out. Secondly—Some 
few of the plants, though perhaps all of living species, are 
supposed to be of genera not now existing in the island. 
They have been described by Sir Charles Bunbury and Pro¬ 
fessor Heer, and the former first pointed out that many of 
the leaves are of the laurel type, and analogous to those now 
flourishing in the modern forests of Madeira. He also rec- 
them the leaves of Woodwardia radicans^ 
ognized 
among 
