OF A PAKT OP COSTA RICA.
333
A few miles to the west of
Cartago, and near the base of the
•volcano Irazu, the usual country
rock is hidden from view, and
replaced by a greyish - looking
rock, which is coarsely crystalline
and rough, and unpleasant to
handle, and in which the felspathic
crystals predominate (No. 5 in
the Appendix, p. 338, is a variety
of this rock).
The felspars are chiefly oligo-
clase; and both grains and crystals
of quartz were observed, also a
little sanidine, augite, magnetite,
and some iron oxides. The rock
is a trachyte, and most probably
a quartz-trachyte. It is known
in the country as the " Cartago
Stone," owing to many of the
churches and public buildings
being constructed of it.
In the several ravines to the
south-east of the volcanoes Irazu
and Turrialba, deposits of sand-
stone are found, containing slight
traces of decomposed felspar.
In ravines and on the moun-
tain-slopes of the volcanoes Irazu
and Turrialba a trachytic rock is
frequently found much finer-grain-
ed than the "Cartago" rock, but
containing less quartz than the
coarser crystalline variety. It is
probably a quartz-trachyte.
Amongst the large boulders of
trachytic rock which lie scattered
on the hill and slopes above the
town of Cartago numerous volcanic
nodules were observed, very close-
grained, and of greyish colour,
with specks of visible augite.
Mr. Hudleston describes it as a fel-
spathic tuff, largely made up of
soda felspars (Appendix, No. 7,
p. 339). Several nodules were
observed about 4-1- feet in diameter.
They are evidently ejectamenta
from either Irazu or Turrialba.
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