150 EARTH FEATURES AND THEIR MEANING 



gases in water. This water, derived from rain or snow, percolates 

 into the ground or descends along the crevices in the rocks, carry- 

 ing with it a certain measure of dissolved air. This air differs 

 from that of the surrounding atmospheric envelope by containing 



relatively large amounts of oxygen and 

 of the other active element carbon diox- 

 ide. It follows from the important role 

 thus performed by the percolating water 

 that the process of decomposition will 

 be relatively important in humid re- 

 gions where the atmospheric precipita- 

 tion is sufficient for the purpose. 



Within hot and dry regions there is- 

 a larger measure of rock disintegration, 

 and distinct chemical changes unlike 

 those of humid regions take place in the 

 higher temperatures and with the more 

 concentrated saline solutions. The dis- 

 cussion of such changes will be deferred 

 until desert conditions are treated in 

 another chapter. 



Mechanical results of decomposition 

 spheroidal weathering. From an 

 earlier chapter it has been learned that 

 the rocks of the earth's outermost shell 

 are generally intersected by a system of 

 vertical fissures which at each locality 

 tend to divide the rock into parallel and 

 upright rectangular prisms. It is these 

 joints which offer relatively easy paths 

 for the descent of the water into the 

 rocks. In rocks of sedimentary origin 

 there are found, in addition to the vertical joints, planes of bed- 

 ding originally horizontal, and in the intrusive and volcanic rocks 

 a somewhat similar parting, likewise parallel to the surface of the 

 ground. The combined effect of the joints and the additional 

 parting planes is thus to separate the rock mass into more or 

 less perfect squared blocks (Fig. 155, upper figure) which stand 

 in vertical columns. 



FIG. 155. Successive dia- 

 grams to show the effect of 

 decomposition and resulting 

 disintegration upon joint 

 blocks so as to produce 

 spheroidal bowlders by 

 weathering. 



