‘62 Scientific Proceedings, Royal Dublin Society. 
dry season to make the murmuring accompaniment one always 
hears in the Alps, and as no breath of wind disturbs the constant 
calm of the air, there is an absolute silence in the forest which is 
very impressive ; even the footfalls of one’s own mules are 
deadened by the carpet of pine-needles. 
These mesas are composed of almost level sheets of light- 
coloured trachyte, and beds of white friable voleanic ash, with 
local layers of white pumice ; incrustations of red and white 
chalcedony, and crystals of celestine are also commonly seen. 
The commencement of the arroyos which feed the Fuerte river 
is usually a basin-like space of bare stone completely denuded of 
all soil, and without vegetation; the sides are steep, and the 
hollows soon plunge down to the depths of the great cafions; 
they remind one of the streams on the peat-covered Irish hills, 
Kippure for example, where a furrow in the living fibrous covering 
of heather and other plants, soon widens into a gully in the soft 
peat. Though the scale of the phenomena is very different, yet 
the forest on the mesa seems to play the part of the heather on the 
peat, and prevent denudation of the underlying material. In the 
basins and valleys there are often rounded bosses, and sometimes 
high pillars left standing in isolated positions, or in groups; they 
appear to be produced by the protection afforded by hard, resistant 
patches in the trachyte ; but, unlike ordinary earth pillars, do not 
develop into symmetrical conical forms; being always twisted 
into strange and odd shapes, and sometimes overhanging consi- 
derably ; some are more than 100 feet high. Besides the summer 
rains, which are not heavy at elevations of seven or eight thousand 
feet, there is a winter snowfall of about two feet, and the greater 
part of this, probably finds its way through the surface soil, and, by 
subterranean passages of the fissured rocks, to the water-level at the 
bottom of the gorges; for in early summer the small streams are 
quite dry, though there is plenty of water far down in the gorges. 
The most striking feature of the mesa on the way from Bocoyna is 
the great Barranca of the Urique river; one approaches it through 
level forest to the very edge of the chasm, where it bends suddenly 
from H.N.E. to8.8.W. The sides are a series of vertical preci- 
pices, formed by the edges of the different lava flows, connected by 
steep slopes of detritus covered with vegetation suited to the eleva- 
tion; for the descent to the stream being 4500 feet, bananas, 
