632 DYNAMICAL GEOLOGY. 



of loose drift-sand, near the sea. In Norfolk, England, between Hun- 

 stanton and Weybourne, the drift-sands have travelled inland, with 

 great destructive effects, burying farms and houses. They reach, how- 

 ever, but a few miles from the coast-line ; and, were it not that the 

 seashore itself is being undermined by the waves, and is thus moving 

 landward, the effects would soon reach their limit. 



In the desert latitudes, drift-sands are more extended in their effects. 



4. Sand-scratches. — The sands carried by the winds, when passing 

 over l-ocks, sometimes wear them smooth, or cover the surface with 

 scratches and furrows, much like glacier scratchings and ploughings, as 

 observed by Wm. P. Blake over granyte rocks, at the Pass of San 

 Bernardino in California. Even quartz is polished ; and garnets 

 are left projecting, upon pedicels of feldspar. Limestone is so much 

 worn as to look as if the surface had been removed by solution. The 

 glass in windows on Cape Cod is worn through by the same means. 

 This principle is now put to practical use, in the grinding and carv- 

 ing of glass, gems, and even granyte, steam being used for blowing 

 a jet of sand against the surface on which designs are to be made, and 

 also, in most operations, issuing with the sand, in order to soften the 

 material. In this way, the deep carvings of a granyte frieze have been 

 made in six hours, that would have required two months of work by 

 hand. 



5. Dust-showers. — Sands are sometimes taken up by whirlwinds or 

 in heavy gales, into the higher regions of the atmosphere, and trans- 

 ported to great distances. 



In 1812, volcanic ashes were carried from the island of St. Vincent 

 to Barbadoes, sixty to seventy miles ; and in 1835, from the volcano 

 of Coseguina, in Guatemala, to Jamaica, eight hundred miles. 



Showers of grayish and reddish dust sometimes fall on vessels in 

 the Atlantic off the African coast, and over southern Europe ; and, 

 when they come down with rain, they produce " blood-rains." Ehren- 

 berg has found that the dust of these showers is to a great extent 

 made up of microscopic organisms. 1 The figures on the adjoining 

 page represent the species from a single shower, which came down 

 about Lyons, on October 17, 1846. The amount which fell at the 

 time was estimated by Ehrenberg at 720,000 lbs. ; and about one 

 eighth consisted of these organisms, making 90,000 lbs. of them. 



The species figured by Ehrenberg include thirty-nine species of siliceous Diatoms 

 (Figs. 1-65); twenty-five of what he calls Phytolitharia (Figs. 66-104), besides eight of 

 Khizopods. The following are the names of the Diatoms. 



Figs. 1, 2, Melosira granulata ; 3, M. decussata; 4, M. Marchica; 5-7, M. distans; 



1 See his work entitled Passat-staub und Blut-regen, 4to, 1847, and Amer. Jour. Sci., 

 II. xi. 372. 



