in places stratification of clay, sand and gravel. Here and there a well is bored and the same story is told. Every valley between these mountain ranges presents the same features, a gentle slope for miles from either side, so gentle that in walking over it one hardly realizes that it is not level, and in the center is—not a stream—but a shallow basin, frequently lined with salty incrustation. Such is the character of the Bolsón, as it is called, which is also a marked feature in the physiography of southern Arizona.
Into these basins the arroyos pour the floods from the mountains. One finds the arroyo in the higher parts of the plain nearer the mountain where the steeper incline gives more velocity and erosive force to the stream. Lateral valleys which lie between high slopes usually develop the arroyo to a marked degree. The deep basin between the ridges, filled with the detrital wash from the slopes, is readily eroded by the swift streams which are produced frequently by the torrential rains of these regions. Running lengthwise through the midst of an apparently level valley floor, these arroyos are often invisible a few feet away, and the traveler may be entirely unconscious of the presence of a ditch thirty feet deep and possibly fifty wide, with perpendicular walls, hardly fifty paces away. The walls of these arroyos are being constantly undermined, and the materials, caving into the channel, are carried out with the rest of the wash; at the same time the head of the arroyo is receding toward the higher land and the channel becomes more and more shallow as the underlying rock comes nearer the surface. As the arroyo extends out into the plain its fall becomes less, the power of erosion by the water decreases and the channel is finally lost on the lower slope.
Thus the composition of the mountains is responsible for the composition of the valley floor. Limestone is the predominating material. Rocks of igneous origin are less conspicuous, but are present, and mineral-bearing veins are plentiful. Occasionally heavy formations of calcareous tufa may be found where springs issue from the hills, as-at the village of Cedros, situated at the end of a short range. In deep and sheltered ditches salts collect on the clay and hang in slender glistening crystals to its surface. The ooze of calcium solutions is everywhere visible in the formation of caliche, which forms a hard, impervious and impenetrable layer on or near the surface of the ground. Here and there it cements together stones and gravel in a solid mass, resistant to weathering and erosion.
But few springs are found and these usually at the foot of the higher slopes. At the western end of a small range, the Sierra del Potrero. water comes to the surface in numbers of strong springs, and on this oasis is built the village of Cedros, the administrative seat of the Hacienda of the Cedars. From the limestone rock, cropping out on the toe of the range, the springs issue forth. One at least of these is warm.