The Botany of Iceland - 01.12.1914, Blaðsíða 30

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THOUODDSEN
streams. The tuffs have usually a brownish-yellow colour owing to
the intermixture of palagonite, a brown dully-lustrous alteration-
product of tachylyte or basalt-glass, which constitutes the greater
portion of the ground-mass of tlie rock; tlierefore the whole forma-
tion is often called the palagonite formation. The tuffs consist of
lava-dust and lava-fragments with an abundance of glass-pieces (pala-
gonite, tachylyte), slag and bombs; loose anorthite-crystals are often
abundant. The breccias are distinguished from the tuffs by being
Fig, 5. Cliffs 011 Klifið (tuff and breccia). The Veslmannaeyjar.
more coarse-grained and containing larger, angular lava-pieces; the
fragments consist of compact basalt, dolerite, pumice, slaggy lava
and volcanic bombs; the separate fragments are often covered with
a glassy crust. The palagonite formation is sometimes arranged in
layers; sometimes not a trace of regular arrangement can be dis-
cerned. The tuffs and breccias have been formed by volcanic erup-
tions which chiefly ejected ashes and lava-fragments, and produced
only a few lava-streams; the ruins of the numerous large volcanoes
from which these eruptions proceeded may still be demonstrated.
The palagonite formation is traversed by thousands of basalt dykes
which branch and send out apophyses and intrusive sheets; very
often the breccia and the tuff is fdled with numerous hollow nodules
and balls of basalt with a radially-columnar structure inside and