40 PRINCIPLES OF STRATIGRAPHY 



must be regarded as a further step in the decomposition of rocks, 

 the silica being separated out and deposited elsewhere as agate 

 or chalcedony. The alkalies likewise are carried away and the sep- 

 aration and redeposition of the iron, if present, also take place at 

 an early moment. 



While in the temperate climates the abundance of humus brings 

 about the reactions which result in the kaolinization of the feld- 

 spars, the absence of this humus tends to subject the feldspars to 

 the attack of non-acidulated waters, which under the influence of 

 tropical heat produce a hydrolytic separation of the silicates with 

 the formation of aluminum hydrate and alkali silicate, which sepa- 

 rates further into alkali ludroxide and free silica. 



InRuciicc of Temperature on Rock Decomposition. 



The influence of temperature on the decomposition of rocks is 

 well shown by the fact that beneath the ever-moist moss cushions 

 of Finland and the northern Urals, the granite shows undecom- 

 posed surfaces, solution being the only process active here, (von 

 Richthofen-zSip^".) This is due to the prevailing high degree of 

 cold. The same thing is seen on the Kerguelen and the Crozet is- 

 lands in the South Indian Ocean, where the cold ht*mid climate has 

 produced but little soil, whereas in tropical regions, as in the Samoan 

 Islands, a deep brilliant red soil, the terra rossa, is produced. The 

 soil of the Samoan Islands is deep and rich and is produced by the 

 decomposition of a basaltic rock similar to that of the Kerguelen 

 Islands, which there, however, is unaffected. (Russell-8o.) 



MOA'EMENTS OF THE ATMOSPHERE. 



WINDS. The movements of the atmosphere are inaugurated 

 by the inequalities in atmospheric pressure and are controlled to a 

 large extent by the rotation of the earth. The normal atmospheric 

 pressure at sea-level, or one atmosphere, is sufficient to balance the 

 weight of a column of mercury in the barometer 30 inches, or 760 

 millimeters, high. As the pressure decreases the mercury falls, as it 

 increases the mercury rises. At an elevation of 5,910 feet above sea- 

 level the mercury stands at 24 inches, at 10,550 feet at 20 inches, and 

 at 16,000 feet above sea-level it stands at 16 inches. (Davis- 

 26:jj.) When air is heated it expands and so becomes lighter, 

 and the same is true when air becomes saturated with moisture. 

 Though temperature and humidity are causes of varying pressure, 

 high temperature and high humidity, producing areas of low pres- 

 sure, and conversely low temperature and humidity, producing areas 



