484 Dr. T. 0. Bosworth — Arid Conditions in S. W. Texas. 



surface of the stone by capillary action when heated by the sun after 

 moisture has sunk into them. 



The Sand and Soil. — The desert sand is derived from the rocks at 

 hand ; it is rather fine and not i:)articularly rounded. It may be 

 a few feet deep, but more often it is only a few inches, and even 

 in the low flat ground bare rocks are here and there exposed. 

 Occasionally sand completely covers a layer of pebbles, but generally 

 it seems to be the presence of pebbles which enables the sand to 

 remain at rest; vegetation may then obtain a footing and allow the 

 accumulation of further sand. Where vegetation retains a hold, 

 patches of rich dark soil are readily formed which is sometimes 3 or 

 4 feet thick. But owing to the wind (and also to the water-flows) the 

 distribution of sand and soil is ever undergoing rapid change. At 

 the ranch mentioned, for instance, an attempt was being made to 

 grow a plot of maize ; at one end of the plot a man was ploughing, 

 but at tlie other end the ground ploughed the day before was so 

 completely covered by sand that no marks of the plough were visible. 

 Again, during the rains, the bare rocks became exposed in places 

 where two days before there had been 3 feet of soil. 



Work of Water. — The land surface is intersected by innumerable 

 watercourses (PI. XVII, Fig. 4) which lead ultimately northward 

 and westward into the River Frio or its tributary, the San Miguel 

 Kiver. During the greater part of the year those rivers are dry, 

 though a little water can usually be obtained by digging in the river- 

 bed. At other times there is a moderate flow of water wandering 

 about in a wide valley which is choked up with a great amount of 

 sand and stones, often disposed in big shoals. These rivers are always 

 overloaded, and at the time of the rains they become torrents bearing 

 an immense load to the sea. The gullies leading int» them are narrow 

 gorges up to, say, 30 feet deep, with steep sides, often vertical. By 

 their complicated windings and the windings of their countless little 

 tributaries where they converge they dissect the ground into ' bad 

 lands ' on a small scale (PI. XVII, Fig. 5). Further from the river 

 the creeks become much smaller, so that the area is mainly cut up by 

 a network of narrow little dry gullies a few feet deep. It was the 

 writer's good fortune to be present in April when the rains came. 

 The creeks filled immediately, and overflowed so that sheets of water 

 spread far and wide, flowing out over the sand and forming large 

 * water spreads '. Fresh watercourses were cut from day to day, so 

 that the system was completely changed. Talus was washed away 

 from the bluffs, pebbles were spread out or washed up into banks, 

 new shoals of sand were formed, stunted trees were uprooted, and soil 

 removed. At intervals between the rain-storms, which usually 

 occurred at night, the sun shone brightly, the waters partly dried 

 up and revealed in the creeks and on the flats fresh sand and mud 

 surfaces all marked with water-ripples and sun-cracks, where 

 previously had been wind-rij^ples or dry sand. 



Ripples. — These water-ripples, whether in the creeks or on the 

 flat, when formed by advancing water present one constant pattei'n, 

 generally in high relief — a kind of horseshoe pattern with all the 

 'shoes' "^ leading forward (PI. XVII, Fig. 6). At bends in the 



