Br. T. 0. Bosworth — Arid Conditions in S. W. Texas. 483 



Denudation and Transportation. — Despite the scantiness of rainfall 

 this is a region of conspicuously rapid denudation ; and tlie transporta- 

 tion of the detritus to the sea is ever visibly in operation. 



The Sun's Heat. — The sun's rays and changes of temperature, as 

 usual in deserts, are causing cracking and crumbling on all bare 

 surfaces, so that debris is continually falling down the sides of every 

 scarp and gully. A curious instance was observed where the sun's 

 heat had had a hardening effect. Here in the freshwater series are 

 certain white clays (comparable with fire-clays) which are very dry 

 and porous, and Avhich can readily absorb much water and become 

 plastic'. In places the occasional rain has washed them, leaving 

 smooth hummocks, and the heat of the sun has then baked these 

 almost as hard as pot, so that they ring when struck with the 

 hammer. 



The Wind. — The wind is usually blowing steadily from the Gulf, 

 and there is a constant visible drifting of material to northward 

 into the valleys of the River Frio and its branches. So effective 

 is the wind that in most cases the talus is disintegrated and 

 removed from the foot of the hills as fast as it can form, and 

 consequently the bluffs are but little protected, and steep scarps are 

 the rule. 



Pehhles. — The fragments of sandstone, calcareous sandstone, and 

 claystone seldom survive even as small pebbles at any distance away 

 from the cliffs, but are quickly reduced to their component grains. 

 Nevertheless, the desert surface generally is littered over liberally 

 with certain small pebbles which decrease in size at a distance from 

 the bluffs, until in the flat plains they are about the size of beans. 

 In places they are some inches apart, in others they almost conceal 

 the sand (PL XYI, Fig. 2). These pebbles in this district are 

 a peculiar assortment, for they consist almost all of chert, agate, 

 chalcedony, and opal. The pebbles are cut so as to have many 

 facets and many sharp edges and corners, and are highly irregular 

 in shape : indeed, they resemble broken flints. IS'one were found 

 of the typical dreikanter pattern, though all are most highly 

 polished and almost lustrous (PI. XYI, Fig. 3). Approaching 

 the bluflPs they are larger and have less facets, and some can be 

 found whose undersides are fractured surfaces which have not yet 

 been presented to the wind. The source of this chalcedonic material 

 is the marine series which caps the bluffs. In those rocks there 

 are numerous fossils, most of which (corals, gasteropods, etc.) are 

 preserved beautifully in semi-opalescent silica, many cracks in the 

 rocks also being occupied by this material. The wide-spread pebbles 

 probably indicate the former extension of the series over many miles, 

 and are evidence of the considerable denudation effected under present 

 conditions. Frequent also among the pebbles are larger stones 

 consisting of silicified wood derived from the freshwater series. 

 Some of the grey calcareous sandstones which form pebbles at the 

 foot of the cliffs are somewhat curiously affected. Outside they 

 have a white shell-like coating up to iin. thick (which is soluble in 

 hydrochloric acid), and underneath this is a reddish coating of iron 

 oxide. These materials presumably have been drawn out to the 



