DISSONANCE OF GREAT PLAINS STRATIFICATION 705 



strictly Miocene character was lost and later formed terranes were in- 

 cluded. In Kansas the term "Plains marls" was appropriately applied 

 to the deposits by Hay,*^ indicating the dominating litholigic feature. 



Of a maximum thickness of over 1,500 feet which the deposits display, 

 the lower half is prevailingly sandy, while the upper half is prevailingly 

 loamy. No other subdivisions are yet recognizable over areas of any 

 considerable extent. It is possible that none ever will be differentiated. 

 The alternation of fine sands and clays, the cross-bedding of the former, 

 the abrupt transition from one to the other, the unusual frequency of 

 unconformities, the remarkable uniformity of physical characters, in 

 spite of the great difference in determined geologic age of the different 

 parts, are stratigraphic points of great interest that demand adequate 

 explanation which hitherto has been denied them. 



In the comparison of the geological structure of the Great Plains with 

 that of other desert and semi-arid plains in other parts of the world, some 

 marked similarities as well as strong contrasts are brought out. Except 

 in a narrow belt in the west the substructure of the region is composed 

 chiefly of unconsolidated, more or less homogeneous deposits which are 

 flat lying and of recent origin. In many of the great intermont plains 

 of the Mexican tableland and of the Great Basin, of the South African 

 plateau, of the Australian interior, and of the Eussian steppes, the geo- 

 logic formations are more or less completely indurated, widely different 

 in texture and composition, tilted and folded and beveled off, and of an- 

 cient date. In the last mentioned cases the plains are now being rapidly 

 eroded; they are areas of degradation. The Great Plains seem to be 

 constantly adding to their volume and height. 



HOMOGENEITY OF PLAINS DEPOSITS 



To one accustomed only to coming in contact with the soils of moist 

 climates, the gravelly surfaces which are often met with in desert and 

 semi-arid areas are apt to give misleading notions of the true character 

 of the materials beneath. Pebbles and rock debris appear to be much 

 more abundant than they really are, because of the fact that in dry cli- 

 mates the finer materials are being constantly removed by the winds.®^ 

 In the case of the Great Plains the occurrence of pebbles in profusion 

 gives rise to the inference that they were deposited there directly through 

 river action. Even the finest and almost pebbleless loams, with thick- 



80 Sixth Ann. Kept. Kansas Board of Agriculture, 1880, p. 02. 



81 Bull. Geol. Soc. America, vol. 10, 1008, p. 73. 



