252 
GEOLOGY. 
appearance of water. The beautiful surface of the pebbly plain may be regarded as a combina¬ 
tion of myriads of reflectors ; for each pebble is so highly polished that it reflects light almost 
like a mirror. The reflection from such a brilliant surface, when seen at a favorable angle, 
looks like a sheet of water ; the similarity being heightened by the motion of the stratum of 
heated air in contact with the surface. 
It is probable that the sand also acts as a reflecting surface, in consequence of the rounded 
form of the grains ; a portion of the incident rays being similarly reflected from every grain, 
and not wholly dispersed in various directions, as they would be if the grains were angular, or 
had flat surfaces. 
The influence which is exerted by climate upon the vegetation of a region cannot be more 
strikingly shown than by a comparison of the deltas, or alluvial lands, of the Mississippi and 
the Colorado. The alluvium of the Mississippi is noted for its fertility and the luxuriance of its 
vegetation. The produce of its plantations enriches States and affects the commerce of the 
world. On the Colorado, a short distance back from the immediate borders of the river, we find 
a broad alluvial plain, with a gray, ashy surface, parched and sun-baked, so that it may be 
quarried in blocks like stone. Scarcely a spire of grass rises above its surface, and there is no 
shade from the burning sun. The air, heated by radiation and direct contact with the clay, and 
unresisted by vegetation, is thrown into violent currents, and sweeps over the plain, bearing 
clouds of dust and sheets of sand with such power and persistence as to abrade and polish every 
obstacle. 
