456 The American Naturatst. [May, 
When a cloud-burst falls upon such a place deep ravines or 
“arroy s” are suddenly formed by the rush of the waters. 
Sometimes an “arroya ” fifteen feet deep is cut out in the soil for 
a distance of a mile. The destructive effects of such a cloud- 
burst are great, particularly when it breaks upon high ground 
and flows upon a plain. In July, 1886, I witnessed the effects of 
such a torrent in the Rio Grande Valley, in New Mexico. The 
cloud-burst broke upon the low Donna Anna mountains, but no 
rain fell outside of their limits. The mountains were capped by 
a dark thunder cloud, while in the valley at Donna Anna, about 
six miles distant, the sky was cloudless. In a short time a wall 
of muddy water a foot high spread over the flood plain of the 
Rio Grande near Donna Anna, and this was followed by similar 
waves for a half-hour. At Donna Anna, which is situated on a 
bluff bounded on the south by an “arroya,” a body of water 
completely filling the arroya was seen to pass by for about a 
half-hour, then the body of water gradually diminished to a 
trickling stream, and in less than an hour no water was flowing. 
The depth of water must have been twenty feet at this point. 
It cut under the cliff on which Donna Anna is built, and scaled 
off a large piece, leaving a house on the edge of a precipitous 
wall. The stream. carried vast quantities of bushes, roots and 
clay, and, I am told, some animals. On the flood plain several 
acres of vineyard were completely covered with silt and gravel, 
one or two small orchards were partially buried, and an adobe 
house about ten feet high was buried to within two feet’ of thé 
top. Several thousand tons of earth must have been removed 
during this torrent. Such floods are not uncommon in these 
arid regions, and their intense erosive activity may be seen by 
this illustration. 
Next in importance to rain is the direct effect of the air as an 
erosive agent. In many places in the arid territories extensive 
sand dunes are proofs of this zolian action; but these are 
merely grand illustrations of a common phenomenon. During 
all months of the year dust is being blown about either in clouds 
or in creeping waves along the surface of the ground. In the 
-valley of the lower Rio Grande, in New Mexico, often for several 


