188 MOUNTAINS OF ARAGUA. 
like slender peaks from a wall of rocks. At two 
in the morning they continued their journey by 
Ortiz and Parapara to the Mesa de Paja. The 
ground over which they travelled forms the ancient 
shore of the Llanos; and, as the chain has now 
been traversed, it may be interesting to present a 
brief view of its geological constitution. © 
In the Sierra de Mariara, near Caraccas, the rock 
is coarse-grained granite. The valleys of Aragua, 
the shores of the Lake of Valencia, its islands, and 
the southern branch of the coast chain, are of gneiss 
and mica-slate, which are auriferous. At San Juan 
some of the rocks were gneiss passing into mica-slate. 
On the south of this place the gneiss is concealed 
beneath a deposite of serpentine, which, farther 
south, passes into or alternates with greenstone. 
This rock is now the principal one, and in the midst 
of it rise the Morros of San Juan, composed of crys- 
talline limestone of a greenish-gray colour, and 
containing masses of dark-blue indurated clay. Be- 
hind the Morros is another compact limestone con- 
taining shells. The valley that descends from San 
Juan to the Llanos is filled with trap-rocks lying 
upon green-slate. Lower down the rocks take a 
basaltic aspect. Farther south the slates disappear, 
being concealed under a trap-deposite of varied ap- 
pearance, but assuming an amygdaloidal character, 
and on the margin of the plain is seen a formation 
of clinkstone or porphyry-slate. 
The travellers now entered the basin of the Llanos. 
The sun was almost in the zenith, the ground was at 
the temperature of 118° or 122°, and the suffocating 
heat was augmented by the whirls of dust which 
incessantly arose from the surface of the steril soil. 
