File:The Quarterly journal of the Geological Society of London (13860240124).jpg

332
G. ATTWOOD ON THE GEOLOGY
Yig. 4. — Sketch of Augite-Andesite with Country Rock adhering to it,
from the surface near the San Rafael tunnel in the Aguacate
Mountains.
of coal have been discovered. The coal is in three conditions

— one,
in which the coal is compact, and in which there is little trace of
vegetable structure

another, in which the plants are partially
carbonized; and a third, which shows the structure of plants (pro-
bably dicotyledonous). The coals may be called lignites

they are
highly pyritized, and contain in many places beautiful white crys-
tals of melanterite, also pieces of jet.
Near the coal, and often adhering to it, are found bands of horn-
stone or chert, of a yellowish-brown colour.
The coal-deposits have not been explored thoroughly, and but
little is known at the present time about their extent. From a
knowledge of the contour of the country and of the surrounding
rocks, I consider it probable that the coal-deposits extend over a
limitod area only.
We now come to a series of ancient lakes, now valleys, whose
waters have been tapped by the Rio Grande, on the Pacific slope,
and by the Rio Reventazon on the Atlantic slope (fig. 5). The old
water-lines on the mountain-sides, and on the hills, once small islands,
in the valleys of Alajuela, San Jose, and Cartago, are very marked
and distinct.
The country rock is the same as above, and continues to within a
few miles of Cartago. Augite-andesite boulders are found on both
sides of the dividing ridge

and in the valleys of San Jose and
Cartago boulders of trachytic rocks are met with.
Large deposits of a fine-grained soft stone are found collected in
some of the ravines near San Jose and Cartago, having a general
white ground, but tinged with yellow, red, and magenta, in fantastic
forms. This was found to be a silicate of alumina highly im-
pregnated with iron oxides; in places the stones are kaolinized,
and resemble lithomarge.
Near this latter rock are found, more or less connected with it,
masses of hard white porcelain-clay, resembling kaolin, or even