- : very d dry. 
178 BOTANICAL GAZETTE [SEPTEMBER 
concluded from this that previous to the deposition of these beds 
there was here a deep trough, which other evidence shows to have 
been the bed of a large river, which existed at a period of greater 
precipitation and was subsequently filled with stream-borne 
material. On account of its fluviatile origin the material is 
extremely variable, being composed of layers of sand, adobe, clay, 
gravel, boulders, and combinations of these materials, with marked 
local variations, both horizontally and vertically. On this mesa 
and the numerous arroyos which dissect its edge grew the plants 
studied. 
One of the principal features of the soil in the habitats described 
by CANNON is a thick layer of hardpan, or caliche, beginning at a 
depth of about 30 cm. and extending indefinitely. This is so hard 
as to prevent root penetration, except through cracks. Such 
layers are common in arid regions, and are formed, according to 
the opinion of CANNON and others, through the concentration of 
salts left by the evaporation of ground water gradually ascending 
by capillarity. Rainfall dissolves these materials, carrying them 
downward. These two processes result in the formation of a 
gradually increasing zone of precipitation. In the Albuquerque 
region this zone is very poorly developed, often being noticeable 
in a fresh exposure only by the presence of a whitish streak or 
pebbles stained with lime. When dry, such soil becomes very 
hard, and it is evident from the appearance of roots entering it 
that it offers considerable resistance to root penetration, but 
does not prevent it. When wet the hardpan is soft and easily 
_ penetrable. — natural conditions, however, it is generally 
is | | arid than that at Tucson, ~ 
= 4 an average fic 10 years | giving a precipitation of 7.44 in., as com- 
3 pared with an average of 11.17 for. Tucson. Much of this small 
“rains in the summer oe tes ones ———: from. the : 
“summer rains. a i a 
