. Elements of geology : a text-book for colleges and for the general reader. Geology. 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 decomposi
. Elements of geology : a text-book for colleges and for the general reader. Geology. 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 wou
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1892
