64 Scientific Proceedings, Royal Dublin Society. 
like a map beneath the giants of the mesa; and were it not for 
their steep and intricate forms, the windings of the river valleys 
could be easily traced through the transparent atmosphere. The 
clearness of the air in these regions is remarkable, even for the 
Pacific coast ; the stars at night shine with astonishing brillianey > 
and a lofty mountain of striking form, the Cerro de Alamos, is. 
visible all day at distances over 60 miles, where nothing inter- 
venes, except in the rainy season. To the south-east of Pandura 
is the peak of Metate standing on a spur of the mesa, isolated by 
the erosion of two streams, and surrounded on three sides by im-. 
mense vertical precipices. There is a wide difference between the 
shapes of mountains in this latitude, and the ice-planed forms of 
Europe; as long as the rivers can deepen their beds they do not 
widen the valleys at all, which remain either cations enclosed by 
cliffs, or V-shaped cuttings, usually terminating at the bottom in 
a narrow gorge. On the lower ridges the evergreen flora is more 
varied and abundant ; three varieties of oak, three of pine, and one 
of arbutus are common; the arbutus, with its thick stem, bright 
orange red bark, dark green leaves, and white flowers, is very pic- 
turesque; and the Hnsia roble,an evergreen oak, with thick leathery 
leaves, is very distinct. Lower down, below 5000 feet, agaves, ma- 
millaria, and prickly pears (Opuntia) are abundant among the grass 
that covers old geological formations, or under the dwarf forest 
that flourishes on recent volcanic ejectamenta. Following the 
ridge towards Guaza, the track overlooks a wide basin, grass-covered 
and dotted with dwarf oaks, into which two streams converge at an 
acute angle, leaving between them a thin slice of table-land, stand- 
ing up perpendicularly 3000 feet above the stream, and revealing 
its structure of horizontal beds; the upper pale acid lava, and the 
lower of dark basic materials. Such wedges are common at the 
margin of the mesa, and are sometimes isolated into towers that 
eventually crumble into conical forms; when first isolated, they 
are locally called caballos (horses); the ridges produced by erosion 
often terminate in a diminishing row of peaks that have been 
formed in this way. This flamboyant style of mountain sculp- 
ture shows that no severe earthquakes have affected this part of 
Mexico for a long time, as many of the grotesque rock-pillars and 
lofty partition walls have such slender bases that a violent shock 
would certainly overturn them. 
