often assume the most singular forms; I saw a pedestal of the earthy
trachyte, covered by a hemispherical portion of a vein, like a great
umbrella, sufficiently large to shelter two persons. I have never met with,
or seen described, any veins like these; but in form they resemble the
ferruginous seams, due to some process of segregation, occurring not
uncommonly in sandstones,--for instance, in the New Red sandstone of
England. Numerous veins of jasper and of siliceous sinter, occurring on the
summit of this same hill, show that there has been some abundant source of
silica, and as these plate-like veins differ from the trachyte only in
their greater hardness, brittleness, and less easy fusibility, it appears
probable that their origin is due to the segregation or infiltration of
siliceous matter, in the same manner as happens with the oxides of iron in
many sedimentary rocks.

SILICEOUS SINTER AND JASPER.

The siliceous sinter is either quite white, of little specific gravity, and
with a somewhat pearly fracture, passing into pinkish pearl quartz; or it
is yellowish white, with a harsh fracture, and it then contains an earthy
powder in small cavities. Both varieties occur, either in large irregular
masses in the altered trachyte, or in seams included in broad, vertical,
tortuous, irregular veins of a compact, harsh stone of a dull red colour,
appearing like a sandstone. This stone, however, is only altered trachyte;
and a nearly similar variety, but often honeycombed, sometimes adheres to
the projecting plate-like veins, described in the last paragraph. The
jasper is of an ochre yellow or red colour; it occurs in large irregular
masses, and sometimes in veins, both in the altered trachyte and in an
associated mass of scoriaceous basalt. The cells of the scoriaceous basalt
are lined or filled with fine, concentric layers of chalcedony, coated and
studded with bright-red oxide of iron. In this rock, especially in the