554 PRINCIPLES OF STRATIGRAPHY 



the larger grains rounded. Others, however, carried in from the 

 desert, are well rounded and assorted. 



Stratification and Cross-bedding of Eolian Deposits. Eolian de- 

 posits may or may not be well stratified. Deposits of dust of a 

 loess-like character often show an entire absence of bedding planes, 

 the material being homogeneous throughout. Walther found in the 

 deserts of Turkestan near Askabad layers of loess-like material a 

 meter or more in thickness without apparent bedding, interpolated 

 between layers of stratified sand and gravel. In the steppes sur- 

 rounding the desert, the coarse and rigid stems of the steppe plants 

 check the wind current, which then drops much of its load of dust. 

 Rain will wash the dust from the air and from the leaves of the 

 plants on which it has settled. This accumulates around the plants 

 through which it is protected from further removal by the wind. 

 The same office is performed by the coarse blades of the Buffalo 

 grass (Bucliloc dactyloides, Bouteloua oligostachya) and other 

 plants which abound in the semiarid Great Plains east of the Rocky 

 Mountains in North America. As the dust accumulates the mantle 

 of vegetation will rise to a higher level. The roots of the dead 

 plants, however, will continue to penetrate the deposit until their 

 decay leaves a series of vertical tubes, or a series of carbonaceous 

 streaks penetrating the structureless mass. The tubes may subse- 

 quently be filled by mineral matter, as in the case of many loess 

 deposits, where the filling of the tubes is by calcite, and where their 

 presence is believed to be in part the cause of the remarkable ver- 

 tical cleavage of these deposits. Older fossil examples of this type 

 are also known, one of the best being the well-known Portage sand- 

 stone of Devonic age, which is exposed in the banks of the Genesee 

 gorge at Portage, New York. The fine, uniform, structureless tex- 

 ture of this rock makes it an excellent building stone, though the 

 presence of the vertical tubes, described originally as fucoids 

 (Fucoidcs verticalis), often mars this quality for certain purposes. 

 The loess-like origin of this rock seems undoubted, though the accu- 

 mulation was near the coast, rather than inland. 



Dust carried by wind to regions of more extensive rainfall 

 will be washed from the air by the rain and accumulate as a more 

 or less stratified loess. In like manner dust carried across sur- 

 faces rendered hygroscopic by the presence of salts will adhere to 

 these surfaces and so, as a rule, become finely stratified. 



On the whole, it must, however, be emphasized that regular 

 stratification is not characteristic of eolian deposits. Where strata 

 show fine or uniform bedding water always has played more or less 

 of an active part in its deposition, though the idea of standing wa- 



