62 PRINCIPLES OF STRATIGRAPHY 



and become buried in the sand dunes. (See, further, Chapter 

 XIII.) Wind is a better sorter than water, hence pure quartz sands 

 will be more commonly formed from crystalline rocks in arid 

 regions. When the rock destroyed is a marine or other sandstone, 

 the resulting sand requires comparatively little purification. 



The sands of the Libyan desert are believed to have been de- 

 rived from the destruction of the Nubian sandstone to the south, 

 whence they were transported by wind to their present position on 

 the Cretacic limestone plateaus of the desert. Grains of lime rock 

 from the underlying foraminiferal limestones are noted in this sand, 

 but otherwise it is pure c[uartz and well assorted as to size. 



CONDENSATION AND PRECIPITATION OF AT- 

 MOSPHERIC MOISTURE.* 



The moisture of the atmosphere is condensed and precipitated in 

 the form of frost or of clouds and fogs, rain, snow, or hail. Air is 

 brought to the point of condensation of its moisture by being car- 

 ried to regions of lower temperature, i. c, up the sides of mountains 

 or to higher latitudes, by the influx of a cooler wind, by radiation, 

 and by expansion. Convection currents will carry the air upward 

 to cooler regions, or to regions where it can expand. Such cur- 

 rents are characteristic of the tropical belt of calms where precipi- 

 tation occurs almost daily. 



Dew. When the temperature of the air is lowered to such a 

 point that, with a given absolute humidity, the relative humidity 

 rises to loo, the next step in cooling will cause the precipitation or 

 condensation of some of the moisture. The temperature at which 

 this takes place in any given case is the dezv point. According to the 

 absolute amount of water in the air, the dew point will vary in tem- 

 perature, being high when there is a large amount of water in the 

 air, and low when the amount is small. Radiation on clear nights 

 may reduce the temperature of the rocks and vegetation to such an 

 extent below that of the air, that moisture will condense in the form 

 of dew, if the condensation temperature is above 32° F. (0° C), 

 or in the form of ice particles if the temperature is below that point. 

 Moisture thus condensed has been observed by the author to cover 

 the rocks and vegetation of the summits of the White Mountains 

 with ice crystals in earlyAugust, these crystals, six inches long, jut- 

 ting out into the teeth of the moisture-laden wind from the vertical 



* For composition of rain water see Chapter IV. 



