III. iEOLIAN ROCKS 



This group comprises a small and comparatively insignificant 

 class of rocks formed from materials drifted by the winds, and 

 more or less compacted into rock masses. They are, as a rule, 

 of a loose and friable texture and of a fragmental nature. 

 Many of the volcanic fragmental rocks (tuffs) are grouped here. 



One of the most common results of wind action on the land 

 is the production of sand-dunes — billowy masses of loose sand 

 which, like drifts of snow, gradually change their outlines and 

 creep onward under the restless goading of the wind. 



Such, owing to their superficial nature, recent origin, and 

 loose state of consolidation, are considered more in detail in 

 the chapter on The Regolith, p. 287. On undergoing consoli- 

 dation, these dune sands may give rise to sandstones in many 

 instances indistinguishable from those of aqueous origin, though 

 less regularly bedded. The finely disintegrated shell and coral 

 sand thrown up by the waves on the beaches of Bermuda is 

 caught up by the wmds and drifted inland, forming hills which, 

 in some instances, are 250 feet in height. Through the deposi- 

 tion of lime carbonate in the interstices of the fragments, these 

 become reconsolidated and form thus the drift rock which com- 

 prises a large portion of the mass of the islands above tide level. 



The finely eonuninuted materials ejected from yoleanic vents 

 may be likewise transported by atmospheric currents and, far 

 from their source, again deposited in beds of no insignificant 

 proportions. These, on induration, give rise to fine-grained tuffs, 

 and, where the final deposition has taken place in water, to 

 distinctly laminated, fine white rocks the lithological nature 

 of which can be made out only by means of the microscope. 

 Such are many of the Pliocene sandstones of Idaho and Mon- 

 tana.^ ( See Fig- 2, PI. 28. ) The following analyses of samples of 

 pumiceous tuffs from (I) Marsh Creek Valley, Idaho; and (II) 

 Little Sage Creek, Montana, will serve to show their composition. 



^ On the Composition of Certain Pliocene Sandstones from Montana and 

 Idaho, Am. Jour, of Science, Vol. XXVII, 1886, p. 199. 



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