Chap. III. LIMPIA PORPHYRY. 459 
In this the traveller proceeds through a series of long 
defiles and narrow clefts, till he unawares reaches the 
western side of the range, and the actual summit of the 
plateau on this side of the Eio Grande. 
From the western foot of the chain the water of the 
rainy season runs through this succession of valleys and 
defiles to the plain at its eastern foot, where it loses itself. 
In the dry season, on the contrary, the traveller is de- 
pendent on some insignificant springs and accidental pools 
of water, and the consequence of wandering from the route 
might very probably be death from thirst. 
At the point where we entered the mountains from the 
plain, a lofty mountain rose on our right, at the east foot of 
which horizontal strata of limestone bordered on the por- 
phyry. Whether they belong to the cretaceous or the 
Jurassic formation, both of which are represented here, I 
do not know ; but the porphyry of the Limpia Mountains 
is certainly older than these limestone strata, which I 
noticed in many places covered with pieces of flint. 
On our left commenced a range of porphyritic rocks, 
which, seen from a height, looked like a vault cleft longi- 
tudinally, and, consequently, like a double row of rocks, 
with a deep fissure between. 
In several places there are evident indications that the 
masses of porphyry, of which this whole system is formed, 
have originally spread in horizontal sheets over the under- 
lying rocks, and that similar casts of melted matter have 
occurred repeatedly one over the other. The highest, or, 
at least, the most imposing mountain in this part- 
Whiting's Peak — -with others of a similar altitude near 
it, are capped by sheets of porphyry in their original 
horizontal position ; and below we find successive terraces 
of the same rock in a similar horizontal position. On the 
