42 THE SCIENTIFIC MONTHLY
valley there is a rise of roughly 6,500 feet. This necessarily means great variety of topography.
The biological covering of so diversified a terrain would be highly differentiated even in a region of the earth's surface in which rainfall is ample in amount and uniform in distribution. Here, irregularity and violence of rainfall is superimposed upon irregularity of surface and plays its part as a powerful environmental factor.
Leaving Tucson by any of the chief highways, one finds himself at once in vast stretches of mesa or of rocky slopes, showing everywhere the most striking marks of water action. To find a xerophytic vege- tation in a region where records of water action are so conspicuous is a great surprise to the novice ; yet the one is really the logical consequence of the other.
The twelve inches, more or less, of annual rainfall are divided be- tween a season of gentler winter showers and another of torrential summer rains. The former leave very little evidence of their occur- rence on the landscape. The latter are often very heavy and their eroding power very great In one of these, quite unusual, to be sure, five inches of water fell — an amount constituting about half the total precipitation of that year and nearly equal in amount to the total rain- fall of the dryest of thirty-one years recorded for Tucson.
The rocky hillsides with only scattered vegetation turn all but a small percentage of the water of these summer cloudbursts into the rocky gulches or canyons, which pass it on to the broad sandy arroyas traversing the long bajadas, transforming both, for a short time, into raging torrents which record their depth by the drift, or even large stones, lodged in the branches of the scrubby trees which mark their courses, in some places many feet above the sandy or gravelly floor, where one fries his bacon and spreads his sleeping bag in the dry season. Past the bajada slopes, the water flows over the broad valleys. Often these are of indeterminate drainage — fine examples of sheet flood ero- sion. Thus the less precipitous mountain slopes show long ragged gashes cut through the superficial detrital layer to the solid rock be- neath, while the fine adobe soil of the apparently flat valleys show here and there areas where the sheet waters have last evaporated, alkali or salt spots, where the drainage is inadequate or sharply carved gut- ters, where the flatness is only apparent and the gradient really suffi- cient to give the flowing water considerable cutting power.
With so large a proportion of the total precipitation coming with violence, immediately running off the surface of the steeper slopes and rapidly sinking into the deeper underlying layers in the valleys, physio- graphic evidence of water action and vegetational evidence of its ab- sence are, in a region of intense heat, inevitable.
This division of the annual precipitation into two periods would
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