GEOCHRONOLOGY 163 
is also the variable intensity of deficiency from one region to 
another. For example, what might be exhibited as an extremely 
deficient period in the Rio Grande region will be less severe (at 
least the trees will have much more relative growth) in the Mesa 
Verde region of southwestern Colorado, and the actual dates on 
the two respective periods will agree only in a general way. The 
evidence indicates that we should have a separate ring series for 
each area because there is no single period of equal intensity and 
duration over the entire Southwest. 
It is impossible to determine the major overall trend for either 
the individual region or the entire area at the present time, 
although future studies may allow a partial solution of this prob- 
lem. In other words, we cannot predict what will happen next 
year, or the following year, or during any future year. As long as 
there is enough moisture to support tree growth, we can determine 
the departures from the average, but this average may be at a 
minimum or a maximum or any point in between, although it must 
be within the prescribed amount of rainfall oth which a par- 
ticular species of tree can grow. Pinus ponderosa can grow, for 
example, in areas where the average rainfall is around 14 inches, 
and it also grows where there is as much as 35 or more inches, 
wherein it begins to be crowded out by other species. One area, 
near Bend, Oregon, has a good stand of ponderosa where the 
average annual rainfall is but 12 inches; this growth is possible 
because the moisture comes at the right time of the year and the 
tree is able to utilize all that does fall. From these figures, we can 
say that the average yearly indices can vary from 12 to 40 inches 
at least. Other factors, biotic, edaphic, etc., can influence both 
the maxima and the minima to the extent that these figures must 
be considered as approximations. 
The subjective diagrammatic deficiency period chart, Figure 1, 
gives what I believe to be the best representation of the periods of 
deficiency for five major areas in the Southwest. Two of these 
areas, Durango and northeastern Arizona, are incomplete. From 
this chart, one can see that there is a period from ca. 50 B.c. to ca. 
A.D. 320 during which there were very few of these so-called 
droughts. Beginning with a deficient period around a.D. 320, 
