272 STUDIES FOR STUDENTS 



or where protracted periods of drought are to be expected during cer- 

 tain seasons of the year. In temperate latitudes this corresponds 

 roughly to between ten and twenty inches of annual rainfall. Under 

 such conditions, as illustrated over the High Plains of the United 

 States and much of the Cordilleran province, the humus is oxidized 

 out of the soil more rapidly than in more rainy climates, being defi- 

 cient in amount but rich in nitrogen; the iron is not leached or concen- 

 trated, except in the presence of occasional shallow lakes or swamps ; 

 the soil is uniformly high in potash, lime, and magnesia. The potash 

 is, moreover, largely in the form of comminuted orthoclase, giving less 

 plasticity to the finer elements of the soil.^ Very little distinction is 

 to be noted between the soil and subsoil of alluvial plains. 



The swampy portions of the flood plain largely become dry during 

 the long dry season, excluding fishes and offering favorable 'breeding- 

 places for mosquitoes during the times that the swamps exist. The 

 delta regions of subarid climates are consequently particularly malari- 

 ous. As examples of such deltas may be cited those along the north 

 shores of the Mediterranean. The Gedis, flowing into the Gulf of 

 Smyrna, possesses extensive delta swamps dry during the summer,^ 

 while the small proportion of swamp in the case of the Nile delta in 

 a truly arid climate is to be compared with the extensive swamps of 

 the Mississippi. Other factors besides climate, however, may assist in 

 governing the ratio of swamps in the latter cases. The thorough 

 seasonal oxidation which is thus allowed of nearly all deposits except 

 those made in permanent water bodies should result, upon their 

 incorporation into the geological record, in a marked dominance of 

 deep-red and brown shales and sandstones, a moderate amount of 

 variegated shales, confined almost entirely to the marginal portion 

 of the deposit, and few or none holding carbon. Lime will exist 

 disseminated in noticeable amount through both shales and sand- 

 stones and may occasionally give rise to markedly nodular or solid 

 calcareous strata. ^ The microscope should show some muscovite 

 and in addition a noticeable amount of feldspar in the finer portions 



^ E. W. Hilgard, Soils, 1906, chaps, xx, xxi. 



2 G. R. Credner, "Die Deltas, ihre Morphologic, geographische Verbreitung und 

 Entstehungsbedingungen, Petermann's Mitth. Ergdnzungshejt, No. 56, Vol. XII, 

 1878, Plate I. 



3 Medlicott and Blanford, Geology of India, 1879, Part I, chaps, xvi, xvii. 



