SOIL MOISTURE AND TO EVAPORATION. 7 



level mesa with its creosote bushes (Covillea tridentata) and several 

 arborescent species of Opuntia; up into the lower slopes of the moun- 

 tains, sparsely covered, like the Tumamoc Hill, with giant cacti, palo 

 verde, cat's claw, and both arborescent and prickly-pear forms of 

 Opuntia; still up into the intermediate region of scattered oaks, agaves, 

 and yuccas, with the beginnings of a real undergrowth of smaller 

 plants; and finally into the true forests of the high mountains this 

 series of transitions would form as instructive a subject for ecological 

 inquiry as can be afforded anywhere. It was with a distinct feeling of 

 regret that the author returned from a reconnaissance trip through the 

 area of these transitions to take up the more definite problems on 

 Tumamoc Hill. 



SOIL STUDIES. 

 GENERAL CHARACTER OF THE SOIL. 



The shoulder of the hill on which the Desert Laboratory is situated 

 rises to an elevation of about 90 meters above the level of the broad 

 mesa below. The mesa surrounds it on all sides, excepting at the 

 south, where the shoulder connects with the flat-topped mountain itself, 

 which attains an elevation of about 200 meters above the plain. The 

 Laboratory building is thus located about midway between the base and 

 the top. 



The mountain is composed mainly of volcanic rock broken into frag- 

 ments on the surface and darkened by weather to a deep brown or 

 black. On the slopes the pockets and crevices between these rock frag- 

 ments are filled near the surface with a heavy brown clay soil. On 

 the gently sloping and practically flat portion of the shoulder just above 

 the building this soil makes up most of the surface, the superficial rock 

 fragments being here not so numerous nor so large as on the slopes. 

 Even in those places which have the deepest soil, however, the pickaxe 

 and spade very soon reach either the bed-rock of the mountain or masses 

 of rock too large to be readily removed or excavated around. Thus deep 

 diggings are almost, if not entirely, impossible without penetrating the 

 rock itself. 



On the mesa below the hill the surface soil is much more sandy and 

 gravelly and few large fragments of volcanic rock are found near the 

 surface. But this soil is underlaid at a depth of a meter more or less 

 by a curious hard-pan of soft and more or less fragmented limestone 

 called "caliche." This is not so hard but that it can be excavated with 

 a pickaxe and is quite permeable to water, although it certainly hinders 

 the downward flow of the latter to a considerable extent. 



