550 PRINCIPLES OF STRATIGRAPHY 



transported for a considerable distance. In the course of long 

 periods of time, deposits of considerable extent and thickness may 

 accumulate, though at present no data are available by which their 

 extent can be measured. 



An important contribution of clastic material available for eolian 

 work is due to the explosive activities of volcanoes. The finer 

 grades of pyroclastics are spread far and wide, and furnish one of 

 the most significant of eolian deposits. As has been shown in an 

 earlier chapter, some of this material is held suspended in the air 

 for a long period of time, and settles far from the place of its pro- 

 duction. The dust from Krakatoa encircled the globe several times 

 before settling. Material which has thus been carried to a distance 

 from the source of production, although in point of origin belong- 

 ing strictly to the pyroclastics, must, in view of its assortment and 

 transportation by wind, be regarded as atmoclastic, since the mode 

 of deposition, more than the mode of production (clastation *), 

 gives the resulting rock mass its chief structural characteristics. 



It should finally be noted that material for eolation is also sup- 

 plied by endogenetic processes. Thus certain pyrogenics, the finer 

 grained lapilli, may be subject to wind transportation and assorta- 

 tion, and on deposition may acquire a typical eolian structure. They 

 would commonly be admixed with and not separable from ordinary 

 eolian pyroclastics. The most typical of atmogenic deposits, snow, 

 is also most commonly exposed to eolian modification, snow dunes 

 or snow drifts being among the common forms assumed by this de- 

 posit. Besides the form, this deposit also shows the other common 

 characters of wind-drifted material, such as uniform size of grains 

 and dune bedding structure, though the latter is brought out only 

 when dust layers alternate with the layers of snow. The ability of 

 the wind-driven snow crystals to act as agents of erosion has been 

 noted in an earlier chapter (p. 52). 



Hydrogenic and biogenic deposits may also be found among the 

 material subject to eolation. Thus salt crystals and gypsum flakes 

 separated from lakes or marginal lagoons may be blown away by the 

 wind. The dune- forming oolites of Great Salt Lake, believed to be 

 of phytogenic origin, and the wind-blown deposits of foraminiferal 

 shells in many regions, are examples of the biogenic anemoliths. 

 Finally, accumulations of wind-blown organic fragments, such as 

 seeds, leaves or whole plants, whether microscopic or macroscopic, 

 must be classed among the biogenic anemoliths. These are proba- 

 bly never of great significance. 



See p. 17. 



