28o STUDIES FOR STUDENTS 



clay plates would not usually hold their form while the broad sweep 

 of sand-laden waters should deposit clean sand both under the edges 

 and over the plates. The concavity of the plates thus testifies to aeolian 

 burial and such may be distinguished from mud-cracked flats buried 

 by fluvial action. Since writing a previous article on "Mud Cracks 

 as a Criterion of Continental Sedimentation,"^ evidence has come to 

 hand from widely separated regions indicating the great geological 

 significance of aeolian-filled mud-cracks in the flood plain deposits 

 of arid climates. A. W. Rogers writing from South Africa makes 

 the following statement: 



Kheis, Griqualand West, May 19, 1907. 

 Today .... I happen to be camped on the narrow flood plain of the Orange 

 River which is a fine place for the observation of mud-cracks. The mud flats are 

 exposed for months and even years before being covered again by water. The 

 mud is a gray-brown silt and the cracks become filled with the deep red Kalahari 

 sand from the north. In places the sand advances as fairly high dunes and buries 

 the mud deeply. Generally, however, smafl thicknesses of the sand are laid down 

 on the mud.^ 



Isaiah Bowman, writing from South America, makes the following 

 statement : 



Iquique, Chili, May 4, 1907. 



Along the inner edge of the Desert of Tarapaca, roughly between the towns 

 of Tarapaca and Quillagua, Chfli, the piedmont gravels, sands, silts, and muds 

 extend for over a hundred miles, flanking the western Andes and forming a transi- 

 tion belt between these mountains and the interior basins of the coast desert. 

 The silts and muds constitute the outer fringe of the piedmont and are inter- 

 rupted here and there where sands are blown upon them from the higher por- 

 tions of the piedmont or from the desert mountains and plains on the seaward 

 side. Practically no rain falls upon the greater part of the desert and the only 

 water it receives is that borne to it by the piedmont streams in the early summer 

 from the rains and melted snows of the high plateau and mountains to the east- 

 ward. These temporary streams spread upon the outer edge of the piedmont 

 a wide sheet of mud and silt which then dries and becomes cracked, the curled 

 and warped plates retaining their character until the next wet season, or until 

 covered with wind-blown sand. The wind-driven sand fills the cracks in the muds 

 and is even drifted under the edges of the up-curled plates, filling the spaces com- 

 pletely. Over this combined fluvial and aeolian, deposit is spread the next layer 



'■ Joseph Barrell, Journal of Geology, Vol. XIV (1906), pp. 524-68. 

 2 Personal communication to the author. 



