ATMOSPHERIC AGENCIES. 5 



section, we find near the surface perfect soil, generally red clay ; beneath 

 this we find the same material, but lighter colored, coarser, and more 

 distinctly stratified ; beneath this, but shading into it by imperceptible 

 gradations, we have what seems to be stratified rock, but it crumbles 

 into coarse dust in the hand ; this passes by imperceptible gradations 

 into rotten rock, and finally into perfect rock. There can be no doubt 

 that these are all different stages of a gradual decomposition. But 

 closer observation will make the proof still clearer. In gneissic and 

 other, metamorphic regions it is not uncommon to find the rock trav- 

 ersed, in various directions, by veins of quartz or flint. Now, in sec- 

 tions such as those mentioned above, it is common to find such a quartz- 

 vein running through the rock and upward through the superincumbent 

 soil, until it emerges on the surface. In the slow decomposition of the 

 rock into soil, the quartz- vein has remained unchanged, because quartz 

 is not affected by atmospheric agencies. Chemical analysis, also, always 

 shows an evident relation between the soil and the subjacent or country 

 rock, except in cases in which the soil has been brought from a consid- 

 erable distance. 



The depth to which soil will thus accumulate depends partly on the 

 nature of the rock and the rapidity of decomposition, partly on the slope 

 of the ground, and partly on 

 climate. In Brazil, undis- 

 turbed soils are found three 

 hundred feet deep.* When 

 the slope is considerable, as 

 at d (Fig. 1), the rocks are 

 bare, not because no soil is 

 formed, but because it is re- 

 moved as fast as formed; 

 while at a the soil is deep, being formed partly by decomposition of 

 rock in situ, and partly of soil brought down from d. Wherever per- 

 fect soil is found resting on sound rock, the soil has been shifted. 



If rocks were solid and impervious to water, this process would be 

 almost inconceivably slow ; but we find that all rocks, for reasons to be 

 discussed hereafter, are broken by fissures into irregular prismatic 

 blocks, so that a perpendicular cliff of rock usually presents the appear- 

 ance of rude gigantic masonry. These fissures, or joints, increase im- 

 mensely the surface exposed to the action of atmospheric water. Again, 

 on closer inspection, we find even the most solid parts of rocks, i. e., the 

 blocks themselves, penetrated with capillary fissures which allow water 

 to reach every part. Thus the rock is decomposed, or becomes rotten, 

 to a great depth below the surface. But, while the rock is gradually 



* American Journal of Science, 1884, vol. xxvii, p. 130. 



Fig. 1 



