which latter, from being easily fused, and then becoming magnetic, I
presume are hornblende. The greater number of the hills, however, are
composed of a quite white, friable stone, appearing like a trachytic tuff.
Obsidian, hornstone, and several kinds of laminated feldspathic rocks, are
associated with the trachyte. There is no distinct stratification; nor
could I distinguish a crateriform structure in any of the hills of this
series. Considerable dislocations have taken place; and many fissures in
these rocks are yet left open, or are only partially filled with loose
fragments. Within the space (This space is nearly included by a line
sweeping round Green Mountain, and joining the hills, called the Weather
Port Signal, Holyhead, and that denominated (improperly in a geological
sense) "the Crater of an old volcano."), mainly formed of trachyte, some
basaltic streams have burst forth; and not far from the summit of Green
Mountain, there is one stream of quite black, vesicular basalt, containing
minute crystals of glassy feldspar, which have a rounded appearance.

The soft white stone above mentioned is remarkable from its singular
resemblance, when viewed in mass, to a sedimentary tuff: it was long before
I could persuade myself that such was not its origin; and other geologists
have been perplexed by closely similar formations in trachytic regions. In
two cases, this white earthy stone formed isolated hills; in a third, it
was associated with columnar and laminated trachyte; but I was unable to
trace an actual junction. It contains numerous crystals of glassy feldspar
and black microscopical specks, and is marked with small darker patches,
exactly as in the surrounding trachyte. Its basis, however, when viewed
under the microscope, is generally quite earthy; but sometimes it exhibits
a decidedly crystalline structure. On the hill marked "Crater of an old
volcano," it passes into a pale greenish-grey variety, differing only in
its colour, and in not being so earthy; the passage was in one case
effected insensibly; in another, it was formed by numerous, rounded and