468 ARRIVAL AT THE SETTLEMENTS. Book III. 
On the road porphyry again succeeds the limestone of 
the Eagle Mountains. A narrow and winding defile, in 
which first limestone and then a species of porphyry and 
a slaty hornstone are seen, leads to the alluvial terrace of 
the valley, where an extensive formation of gypsum occurs. 
The defile is only three miles long, but the road through 
it is so difficult for large waggons with long teams, that it 
took us ten hours to pass through it. At about midway, 
by the road-side, is a rounded rock covered with Indian 
hieroglyphics. The gypsum in the valley appears as a 
loose earthy mass of red, yellow, white, and greenish 
colour, containing innumerable crystals of selenite in 
laminae, scales, and fibrous aggregates. The gypseous 
clay is destitute of vegetation, whilst the adjacent alluvial 
soil is covered with rich grass. Here, and in the defile, 
I frequently observed the chino-grass (Sacate chino) which 
is celebrated in Mexico, and is prized far more than the 
gramma-grass, and as much as oats : it grows only a few 
inches high. 
By the river we lost several mules, which drank so 
much water that they dropped, and could not rise again. 
We now found ourselves once more in the vicinity of 
human settlements, and despatched people to San Ygnacio, 
to purchase some waggon-loads of maize for our animals. 
Without this supply we should hardly have been able to 
reach El Paso, as they were so exhausted that almost 
hourly one or another of the animals dropped down before 
the waggon, and was with difficulty got on its legs again. 
On the 20th of March we encamped opposite to the 
above-mentioned Mexican village. The inhabitants came 
over the river to bring us poultry, eggs, and milk for 
sale. 
Our condition, as well as the surrounding country, 
