THE LOWER AND MIDDLE SONORAN ZONES IN 
ARIZONA AND NEW MEXICO. 
T. D. A. COCKERELL. 
FoR some years it has been a matter of controversy whether 
the Mesilla Valley (3800 ft. altitude) in New Mexico should be 
regarded as Lower Sonoran. The present writer classed it as 
Upper Sonoran, and later placed it in an intermediate zone, 
which he proposed to call Middle Sonoran. Dr.C. H. Merriam, 
on the other hand, regarded it as true Lower Sonoran, and so 
mapped it. 
The writer, unfortunately, had never been able to study the 
true Lower Sonoran until his visit to Phoenix last year. This 
visit, while undertaken for other purposes, enabled him to 
reach some new conclusions, which are here set forth. While 
the discussion is largely technical, it is hoped that it will arouse 
some general interest, as the conclusions reached are of practi- 
cal as well as theoretical interest. 
We have in New Mexico and Arizona a number of fertile 
valleys, of which the Salt River and Mesilla Valleys are per- 
haps the most important. The Salt River Valley, in particular, 
is said to export more fruit than all the rest of Arizona com- 
bined. Now these valleys, between them, have to supply cer- 
tain markets, and it is of the highest importance to each one 
that it should produce that which grows to a reasonable degree 
of perfection and finds a ready sale. If the valleys all belong 
in the same zone, they may be expected, speaking broadly, 
to produce the same crops, and it may be that in so doing 
they will overstock the market. But if there is in reality a 
great diversity in the agricultural conditions of the several 
valleys, so that they not only differ in detail but belong to 
different life zones, — this is a fact of the utmost practical 
importance, since it indicates that there may be such diversity 
in products as almost to avoid injurious competition. More- 
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