File:The Quarterly journal of the Geological Society of London (13859889323).jpg

OP A PART OP COSTA PICA. 331
Aguacate mines by very crude appliances since they were first dis-
covered in 1822

and the value may be estimated at .£1.200,000.
On the Aguacate Mountains volcanic rock-masses were observed on
the hill-tops, on the sides, and in the ravines. They have a general
north- and- south direction, as may be inferred from their strike.
In the mine-workings, however, which have reached a depth of 400
feet vertical, although several miles of subterranean galleries were
explored, no evidence of any eruptive rock-masses cutting the veins
was discovered, or of their disturbing the same. Numerous galleries
which have been run outside the veins prove, in many cases, the
existence of the two different rocks — the country rock (or conso-
lidated ash) and the augite-andesite, which are often in contact
(fig. 4).
Fig. 3. — Directions of Lodes in the Aguacate Mines.
a, a'. Strike of the Oreamunos-San Rafael-Los Castros lode, N. 41° 30 r E.
b, V. Strike of the San Miguel lode, N. 37° E.
This latter rock presents a great resemblance to the black-
looking boulder-stones found on the hill-sides near the road on the
way from Esparta

and the result of a microscopical examination
of rocks found near the surface in situ, and of others obtained 400
feet below the surface in one of the Aguacate Company's galleries
called the San Eafael Level (Appendix, No. 4, p. 337), prove them
to be almost identical. They have a compact base, are of a brownish-
black colour, contain triclinic felspar, augite, magnetite, specks of
pyrites, and also of ferrite. Petrographically they may be referred
to the augite-andesites, and occur of irregular shape and size, occa-
sionally appearing upon the surface, but often hidden from view.
They never present a wall-like mass of mineral matter filling up rents
or fissures in the original strata.
A sample taken of the rock about 400 feet below the surface,
and near the point of contact with the country rock, shows slicken-
slides in a very marked manner.
No change of geological interest is observed until the river called
Bio Grande is reached. In several ravines near the river