40 WASTE AND RECONSTRUCTION. 



winds will remove it and carry it away to some more shel- 

 tered locality. And if the set of the wind be constant, or 

 chiefly from one direction, like the trades and sea-breezes, 

 the result in the long-run will be very marked and per- 

 ceptible. By this means the dry sand of the sea-shore is 

 blown inland and beyond the reach of the tide into mounds 

 and hillocks (sand-dunes, as they are termed), and along 

 every shore in the world there are recently-formed expanses 

 of this nature, often like the " Landes" of France of vast 

 extent, and still in the process of augmentation. As with 

 the sands of the sea-shore, so with the sands of the arid 

 deserts ; they are driven hither and thither into dunes and 

 ridges, but chiefly forward in one main direction according 

 to the prevailing winds, and this to the obliteration of 

 streams and oases, and to the destruction of fertile valleys 

 that lie in their way. Gentle as it may seem, the drifting 

 of sand over the surface of granite and basalt has been 

 known to wear and polish down their asperities, and even 

 to grind out grooves and furrows like those produced by 

 the long-continued motion of glacier-ice or the flow of run- 

 ning water.* 



But perceptible as may be the effects of the meteoric 

 forces, they are far less obvious than those produced through 

 and by the agency of water. The Aqueous are generally on 

 a larger scale ; and wherever streams and rivers run, waves 

 break and tides ebb and flow, there they are to be witnessed, 

 partly as degrading, but partly also as accumulating and re- 

 constructing forces. The mere passage of water over rock- 

 surfaces would of itself have little effect ; but as it bears 



* At the Pass of San Bernardino in California,, Mr W. P. Blake (as 

 quoted by Professor Dana) observed the granite rocks not only worn 

 smooth, but covered with scratches and furrows by the sands that were 

 drifted over them. Even quartz was polished, and garnets were left 

 projecting from pedicles of felspar. Limestone was so much worn as to 

 look as if the surface had been removed by solution. 



