THE ATMOSPHERE AS A MECHANICAL AGENT. 



161 



156. 



Other views in Mr. Walther's book represent deep excavations in nearly- 

 vertical bluffs, sometimes in regular alternation with narrow columns — the 

 latter the part which descending solutions of some kind (pertiaps calcareous 

 or ferruginous) had hardened ; often they are very irregular in form. 



A blast of sand propelled by steam is now employed (after Nature's sug- 

 gestion) in grinding and 

 carving glass, gems, and 

 even granite. Glass cov- 

 ered by lace-work, or by 

 paper having open pat- 

 terns cut in it, is rapidly 

 worn where its surface is 

 exposed, while the lace or 

 paper, owing to its yield- 

 ing before the sand, shows 

 scarcely any effect of the 

 blast. Large cornices and 

 mouldings of granite are shaped by a blast of steam and sand. 



Thoulet, of Paris, has investigated the effects of air-blast abrasion (1887) 

 and found, besides other results, that moist rock abrades most easily, and 

 that the effect is small if the surface struck has a dip of less than 60°. 



Upturned Cretaceous beds near Abu Roasch. Walther. 



Transportation and Deposition. 



The deep deposits of earth over ancient monuments in Rome and other 

 old cities is largely a result of eolian transportation. The most extensive 

 drift-sand deposits occur over arid areas where there is little or no vegetation 

 to fasten down the sands, and where nearly all the year through the work is 

 going on. But the best known are those of windward shores where fronted 

 by long beaches. The sands of seabeaches often extend out long distances 

 in the shallow waters. The breakers come in sand-laden, to throw the sand 

 up the beach, and in ordinary weather the beach takes the whole. But 

 storm-winds carry the sands from the breakers and the beach over the low 

 surface beyond and pile it into ridges, often making a series of parallel sand- 

 drifts. The sand keeps moving landward with each season of storms, unless 

 stopped by steep declivities, or by vegetation whose encroachment is favored 

 by moist soil ; and sometimes it drifts up the sea-border hills to heights of 

 100 to 200 feet. The surfaces of drifted sands are often covered with 

 ripple-marks. 



The effects are greatest (1) where the sands are fine, and most purely 

 siliceous and therefore incoherent ; (2) where the coasts are well open to 

 the winds ; (3) in regions exposed to the most violent storms ; and (4) espe- 

 cially on projecting points where the work is carried on in succession by the 

 winds of both sides of a rotary storm, and by storms of different directions. 

 Ordinary winds have little effect, and hence on the Pacific coral islands the 

 Dana's manual — II 



