^2 > GEOLOGY. 



surface the solid matter taken from the air. The amount of material, 

 thus added to any given region in any particular shower is trivial, 

 but in the course of long periods of time the total amount of material 

 washed out of the air must be very great. 



Every rain-drop strikes a blow. If the drops fall on vegetation, 

 they have little effect, but if they fall on sand or unprotected earthy 

 matter they cause movements of the particles on one another, and 

 this movement involves friction and wear. While the results thus 

 effected are inconsiderable in any brief period of time, they are not 

 so insignificant when the long periods of the earth's history are con- 

 sidered. 



Clayey soils contract and often crack on drying. Falling on such 

 a soil when it is dry the rain causes it to expand, and the cracks are 

 healed by lateral swelling. The same soils are baked under the in- 

 fluence of the sun, and when in this condition are softened and made 

 more mobile by the falling of rain. Under the influence of the ex- 

 pansion and contraction occasioned by wetting and drying, the soils 

 and earths on slopes creep slowly downward. When rain falls on dry 

 sand or dust the cohesion is at once increased, and shifting by the 

 wind is temporarily stopped. 



After the water has fallen on the land its further work cannot 

 be looked upon as a part of the work of the atmosphere ; but any con- 

 ception of the geological work of the atmosphere which did not recog- 

 nize the fact that the waters of the land have come through the atmos- 

 phere would be inadequate. The work of the water after it has been 

 precipitated from the atmosphere must be considered in another chapter. 



///. Effects of Electricity. 



Another dynamic effect conditioned by the atmosphere is that 

 produced by lightning. In the aggregate this result is inconsequential; 

 yet instances are known where large bodies of rock have been frac- 

 tured by a stroke of lightning, and masses many tons in weight have 

 sometimes been moved appreciable distances. Incipient fusion in 

 very limited spots is also known to have been induced by lightning. 

 Where it strikes sand it often fuses the sand for a short distance, and, 

 on cooling, the partially fused material is consolidated, forming a 

 little tube or irregular rod (a fulgurite) of partially glassy matter. Ful- 

 gurites are usually only a few inches in length, and more commonly 



