164 THE FUTURE OF ARID LANDS 
however, such periods were common and more intense until the 
late 1200’s when a long deficient period occurred. The 1300’s were 
mostly good, as were the 1400’s. Several periods occurred during 
the 1500’s, with a severe one at the close of the century. From 
1600 to the present, conditions have been favorable, and a short 
interval during the turn of the twentieth century 1s about the only 
really deficient period. The horizontal line in the deficiency curve 
at the bottom of the figure has no quantitative meaning; it could 
represent 12 inches, or 40 inches, or any point in between, and it 
could, in itself, be a fluctuating average or curve which it probably 
should be. 
Possible changes in the average annual rainfall are indicated, I 
believe, by the following bits of information. Samples obtained 
from living trees growing on the higher mountain ranges of south- 
ern Arizona rarely have chronologies extending to before A.D. 1600. 
Detailed studies now beginning on species identification of woods 
found in archaeological sites scattered over the Southwest indicate 
that a high percentage of specimens from a site, say dating around 
A.D. 500, are Pinus edulis, yet today the area is well covered with 
Pinus ponderosa. Tentative studies indicate that the presence of 
prosopis in archaeological sites located on the Sonoran Desert, 
seems to be confined to only those sites which are fairly recent in 
time, say the last 300 or 400 years. Charred hickory (Carya sp.) 
was found in the Double Adobe site located about 12 miles 
northwest of Douglas, Arizona. Antevs has placed an age of about 
g,000 to 10,000 years on this site. The presence of hickory indi- 
cates a far more moist climate prevailed than there is at the 
present time. The closest hickory, today, is found in eastern 
Texas, about 800 to 1,000 miles to the east. 
It is interesting to note, although it is beyond the scope of our 
purpose here to go into detailed analysis, that Figure 1 has much 
meaning archaeologically speaking. Studies concerning the correla- 
tion of shifting populations during times of deficient periods of 
rainfall, as expressed in tree rings, are now being inaugurated. 
Evidence based on past studies indicates that there was, during 
such times of stress, considerable shifting of the population centers 
from one region to another. 
