CRITERIA AS TO MODES OP ORIGIN OF SEDIMENTS 359 



During the long dry periods the sand is shifted by each high wind; the 

 small grains acquire a millet-seed roundness foreign to the work of water ; 

 the dust is taken up in the air and exported from the desert basin. 

 Where sand blows across stony floors the pebbles become faceted, giving 

 the sharp-edged and polished forms known as dreikanter. Where sand 

 progressively accumulates through wind action the base of each dune 

 remains behind in the forward marching of the dunes. The dunes of 

 the Sahara are frequently 300 feet in height. Owing to the shifting of 

 winds, all of one series of dunes may be removed elsewhere; a hundred 

 feet of the base of others may remain. Cross-bedding of marked irregu- 

 larity on a gigantic scale is developed from this in the growing deposit. 

 The cross-bedded members of dune sands are not limited by parallel and 

 horizontal surfaces, and the lines of cross-bedding are broad sweeping- 

 curves, tangent to the horizontal at the base and tens or even some hun- 

 dreds of feet in their radii of curvature. 



In true desert deposits there is a marked absence of the argillaceous 

 component, although the sediments of the earth as a whole consist of 80 

 per cent shale. The constituents of shale are borne from the desert 

 region as dust, deposited hundreds of miles away as loess, or mixed with 

 the deposits of more humid climates, or with the limy oozes of open seas 

 or deep permanent lakes. 



Where large rivers maintain their way across deserts or terminate 

 within them the alluvial deposits are very largely reshaped by wind, as 

 over the delta of the Indus, and notable deposits of gypsum and salt mark 

 the sites of lagoons or interior seas. 



Contrast the preceding with the conditions of sedimentation under 

 semi-arid or seasonally dry climates. This is an entirely distinct cate- 

 gory, yet one which is practically not recognized by Walther, the apostle 

 of deserts. Climates of this character are marked by a concentrated 

 rather than a deficient rainfall. If the temperature of the wet season is 

 not too cold, an abundant vegetation may grow and a rich and varied 

 fauna may inhabit the land. Such climates exist over much of the 

 tropics, as seen in Africa and India ; over broad continental interiors in 

 the temperate zones, as seen in the great wheat and cattle lands of the 

 globe. 



Under semi-arid climates water is the great transporting agent. Dur- 

 ing the rainy seasons the shrunken rivers fill again, flow long distances to 

 the sea, sweep gravel far from the hills, wash out the evaporation deposits 

 of the dry season, cover up the mud-cracked alluvial flats with new lavers 

 of silt and clay- During the dry season the herbaceous vegetation turns 

 brown, the humus is oxidized out of the soil, the slimes left by the last 



