Dr. T. 0. Bosworth — Arid Conditions in S. W. Texas. 485 



course of tbe current the 'horseshoes ' are obliquely distorted.^ The 

 wetted surfaces, having the appearance of mud flats, are rapidly 

 cracked up by the sun's heat into hexagonal patterns with wide 

 cracks. Drying proceeds rapidly and sometimes surface films of the 

 hexagons may be seen curling up, each into a thin roll, and then 

 crumbling up into ordinary fine quartz sand. Remarkably rapid 

 was the effect of the sudden rains upon the vegetation. The stunted 

 mezquit bushes put forth abundant light yellow-green leaves, and in 

 a day or two the ground was ablaze with bright-coloured flowers, so 

 that in every direction as far as the eye could see the earth appeared 

 red or blue or yellow, or of varied hue, as the case might be. (These 

 small plants display much flower and very little leaf.) A multitude 

 of small Gasteropods feed upon this vegetation. 



Gypsum. — Gypsum is being formed almost everywhere. In the 

 floors of the gullies are occasional growths, several feet across, 

 composed of spheralitic structures, the spherules being upwards of 

 several inches in diameter and built of radiating fibres. On all talus 

 slopes where there is argillaceous material, thin sheets and slabs of 

 gypsum are ever forming. They appear to grow a few inches beneath 

 the surface, and also in cracks. The slabs are formed of fibres normal 

 to the plane, and are usually only a fraction of an inch in thickness, 

 though some are thicker. Thei-e are also single larger crystals of 

 selenite of the usual form. It appears that gypsum grows very 

 readily in argillaceous matter which is much exposed to the sun's 

 heat and is occasionally moistened. 



Desert Deposits. — Although the whole region is overspread with 

 sand, yet nowhere were any thick deposits observed to be forming. 

 Indeed, the resultant effect of wind and water in this area is gradual 

 denudation, with intermittent sudden transferences of material to the 

 Hiver Frio, and thence by sudden impulses to the sea. The deposit 

 due to the denudation in this desert is a delta cone in the Gulf of 

 Mexico, and it is only there that the blown sand and the multitude 

 of hard wind-cut stones come to a lasting rest. 



EXPLANATION OP PLATES XVI AND XVII. 

 Figs. Plate XVI. 



1. Characteristic vegetation of the arid regions of Texas, consisting of: 



'pricMy-pear cactus' (Opuntia vulgaris), 'Spanish dagger,' the 

 large aloe (Yucca aloifolia), bushes 'of 'mezquit' (Prosopis 

 glandulosa) , and ' chaparal ' (thick bramble-bushes entangled with 

 thorny shrubs in clumps). 



2. Sand and stone-covered plains (typical view). 



3. Wind-cut pebbles on the desert (mainly chalcedony). 



Plate XVII. 



4. A dry watercourse. 



5. Tributary entering the Eiver Frio valley (the foreground is one bank of 



the tributary). In the background is the Frio valley. 



6. After the rains. The surface shows the form of the ripples left by 



a water-flow ; all of horseshoe pattern, with the ' footmarks ' pointing 

 forward. 



■^ Earely (as in some places where steady flows lapped a sandbank) a more 

 linear type of ripple was seen on sloping surfaces. Also linear forked ripples 

 were formed occasionally in still pools where the water was agitated by wind. 



