14 STRATIFICATION. 



coarse limestone in its turn covers a stratum of plastic clay ; in 

 many places where the coarse limestone is not very thick, it has 

 been pierced through, and the plastic clay found beneath it. 



13. But it is not necessary to dig wells in order to be certain of 

 the superposition of the different layers formed by these rocks ; it 

 is distinctly seen by examination of the declivities of certain hills, 

 or cuts made through them for the passage of roads, &c. ; for, 

 when the point of contact of two layers is exposed at one of these 

 localities, we may frequently distinguish, without difficulty, the 

 manner in which one of these layers is continued beneath the 

 other. 



14. In other places nothing similar is seen ; the rocks show 

 no trace of stratification, but constitute compact masses, such as 

 granite. 



To form an idea of the manner in which nature has produced 

 these immense earthy layers, we must study the phenomena which 

 are now taking place at different places on the surface of the 

 earth. 



15. The action of rain, of the sun, of frost, and many other 

 causes are constantly tending to change the surface of rocks, even 

 those which are most compact, and to detach fragments from them ; 

 these fragments, more or less divided, are spread out over the sur- 

 face of the soil, mixed with the detri'tus* of plants and animals, 

 and constitute a kind of movable bed, more or less thin, which 

 covers the whole surface of the globe, and bears, commonly, the 

 name of vegetable earth, because it is in this bed that almost all 

 vegetables grow. The mineral substances which enter into its 

 composition are ordinarily sand, clay, or the debris, or remains of 

 calcareous rocks. 



16. When currents of water pass over movable formations, such 

 as we have just mentioned, they take up a portion and convey to 

 a distance the detri'tus and debris of which they are composed. 

 In this way, when the heaped-up snows on the tops of mountains 

 melt under the influence of the summer's sun, or when abundant 

 rains fall on the same places, impetuous torrents descend towards 

 the plain, and carry with them earth and fragments of stones found 

 in their route, or 'vhich they tear up from their resting-places ; the 



* DETRX'TUS. A geological term applied to deposits composed of various 

 fubstances which have been comminuted by attrition. The larger frag, 

 ments are usually termed debris ; those which are pulverized, as it were, 

 constitute detri'tus. Sand is the detri'tus of siliceous rocks. 

 # : : 



13. What evidence have we of the superposition of strata ? 



14. Are all rocks stratified ? 



15. What are the common causes which tend to change the surface of 

 rocks ? What is detri'tus ? What is vegetable earth ? What is debris ? 



16. How do currents of water change the surface of the earth? 



