350 G. K. GILBERT 
lesson is conveyed by the presence of pillars of earth each capped by 
a pebble or other protective particle, but it is not easy to determine 
whether the work of sculpture consumed little or much time. In 
this particular case the raindrops were driven by so violent a wind 
as to be swept up a slope. The wind in question blew for but a 
fraction of an hour, but in that brief time the rain beat developed on 
the earthbank (of regolith) a complete system of furrows and ridges 
parallel to the direction of the wind. 
