DESERT SCENES IN Z AC AT EC AS 437 



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 Bolson, 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 moun- 

 tain where the steeper incline gives more velocity and erosive force to 

 the stream. Lateral valleys which lie between high slopes usually de- 

 velop 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. Eunning 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 sur- 

 face. 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 compo- 

 sition of the valley floor. Limestone is the predominating material. 

 Eocks of igneous origin are less conspicuous, but are present, and min- 

 eral-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,. 



