532 TOADS AND FKOGS. Book III. 
our animals twice to drink. Water rises in this well in 
a clayey bed of alluvial soil. In the evening we started 
again, and journeyed throughout the night. We now 
first came to the drift-sand hills, which mark the limit 
of the higher desert plateau. In the bright moonlight 
we crossed a tract of sandy downs, on which there was 
no sign of any road visible. After we had proceeded 
several miles, I perceived by the stars, that our caravan 
was unawares taking a circular route, and was actually 
journeying back in the direction of the Colorado. Not 
much ground, however, had been lost, and ere long we 
discovered again the traces of a beaten road. 
In this neighbourhood I perceived a smell of sul- 
phuretted hydrogen, which, I was informed, proceeded 
from a mud-volcano, lying at a considerable distance on 
the northern side of the Colorado, and the vapours of 
which were often carried by the wind far across the desert. 
By daybreak the next morning we arrived at the next 
watering-place, called Alamo Mocho : it is a much deeper 
well than the former one, and is prevented from falling in 
by boards. Together with the water we drew up innu- 
merable large toads and frogs. At this place we met the 
postman of San Diego, a young man of education, from 
Virginia, who, for 100 dollars a month, performs the 
journey weekly to and from San Diego and Camp Yuma. 
The well itself is below the edge of the higher region of the 
desert upon which the road runs. The edge is about fifty 
feet high, and exhibits various alluvial strata, one of which 
consists of a fine hard clay, its fracture shining with a kind 
of pearly lustre. This clay seems to correspond to the 
slippery mud occurring in the Colorado, as well as in the 
first brooks met with at the foot of the Californian moun- 
tains, and which is so soft that, on putting your hand into 
