180 



EFFECTS ATTRIBUTABLE TO EROSION. 



Many other facts are explained by the power of erosion and transport by 

 water. When, by studying faults in the interior of mines, we clearly see 

 that the beds no longer correspond, and that a part of the formation must 

 have been uplifted (Jig. 289) ; then, if the soil, a, fe, c, is level on the surface, 



Fig. 289. Fig. 290. 



we naturally ask what has become of the beds d and f, which ought to have 

 formed a hillock between b and c. It is clear these beds must have been 

 removed, which we may conceive was only by a posterior action of waters, 

 which carried away the debris, and perhaps spread them over the surface. 

 In the same way, when we see a vein form a projection, a dyke on the sur- 

 face of the soil (fig. 203, page 119), we conceive that it could not have 

 formed in this manner, and that the uncovered part must have been once 

 encased just as that is which is now covered ; the surrounding formation 

 has been uplifted then afterwards, at least along the whole actual height of 

 the projection. Something similar necessarily took place at points where 

 veins crop out on the surface, or arc covered by moveable soil (^/fg - . 290) ; it 

 is not probable that melted matter injected in the crack would be immedi- 

 ately arrested at the surface of the earth, and it is presumable that Ihe soil 

 has been removed and subsequently covered by various clearings. We are 

 thus led to understand how so many basa'ltic masses now offer no trace of 

 scoria'ceous matter, neither in themselves nor in their vicinity. These im- 

 perfectly aggregated debris have been subsequently carried away by the 

 action of water, and perhaps it is the same with the scoria'ceous matter 

 which must have accompanied the appearance of trap. 



The prodigious power exerted by waves, and the effects they have pro- 

 duced in our times, lead us to think, also, that all the rocks formed around 

 islands and reefs at a short distance from coasts, or the often fanciful groups 

 in the midst of the sea, are also the remnants of some great division caused 

 by water, as much in removable matters, easily disintegrated, as in masses 

 broken by earthquakes and different movements of the soil, and certain 

 parts of which have been afterwards removed, either by repeated shocks of 

 waves or sudden debacles. In this way we may explain the numerous 

 accidents in rocks which bound coasts, or are isolated in the midst of the 

 ocean, as in the sinkings of the chalk of Etretnt (fig. 291), and the sec- 

 tions of porphyritic or granitic rocks in the Shetland islands (fig. 292). It 

 is conceived that straits, more or less extended, may have been formed 

 by the two combined actions of currents of water and rupture which the 

 soil might have undergone, by upheaval or subsidence, at determined 

 epochs. 



From these observations, we see that many effects may be attributed to 

 the action of water which cannot be in any other way explained. We may 

 see denudations in the midst of mountains and valleys, recognise the ancient 

 sinkings which bordered seas at different ages, and hence appreciate their 

 limits, as well as all other circumstances connected with them. Reference 

 to the immediate action of water should be always carefully restricted to the 

 moveable or loose matters found on the surface of the globe; for when 

 solid matters are in question, which water attacks too slowly, we are led to 



