538 STUDIES FOR STUDENTS 



and may be the seat of a considerable assemblage of living forms. 

 The result is that the lake clays need not be saline, but are hkely 

 to be leached of iron or be even carbonaceous and fossiliferous. Foot- 

 prints may also be common on the shores and the remains of land 

 plants, and animals may become entombed in the deposits. The 

 wash of nearby land waste and the action of waves may fill up the 

 mud-cracks with sand and thus lead to their permanent preservation. 

 Interior seas are unstable bodies whose shores are ever varying, 

 and which are finally destined to be either dried up into playas, 

 the fate of Lake Lahontan, or to be filled up with sediment, giving 

 rise to river flood-plains, the fate at present overtaking Lake Titi- 

 caca, or by becoming fresh to be drained by cutting down an outlet, 

 a change at present in progress in several of the large African lakes. ^ 

 As seen in geological section the mud-cracked margin should be 

 transitional on any one horizon between fine-grained, paper-thin, 

 lake clays, on the one hand, showing no mud-cracks, and the coarser 

 slopes of land waste on the other. In ascending through the forma- 

 tion such mud-cracked shales should oscillate laterally and occupy 

 but a portion of the series. They could hardly, therefore, be a 

 characteristic feature of the sediments in general, nor even of the 

 bodies of shales originating in the lakes of interior basins. 



MUD-CRACKS OF RIVER FLOOD-PLAINS 



Description 0} present conditions.- — Over all river flood-plains 

 inundations periodically take place, and as the flood waters gradually 

 drain away, a large quantity of fine mud is left upon the surface, 

 perpetually renewing the fertility of the soil. Where the climate 

 is humid, as over the delta of the Mississippi, such regions become 

 seats of luxuriant verdure, while on the contrary in arid or semi- 

 arid regions, an evanescent vegetation may spring up following 

 the flood, but as soon as the water is drained away and the level of 

 the ground water sinks beyond the reach of the plant roots the region 

 becomes a desert until the period of the next inundation. Such 

 regions are abundant over the desert belt of the world, the flood 

 plains of Egypt, of Mesopotamia, and of the Indus River being 



I Albrecht Penck, "Climatic Features in the Land Surface, American Journal 

 Science, Vol. XIX, p. 171 (1905). 



