101 



have been made with the view of ascertaining the mean depth 

 of the sea ; this is a mathematical refinement which cannot be 

 supported by physics. We may as well attempt to find the 

 mean height of the dry land above the level of the sea by cen- 

 trifugal forces, &c* However, our present question is that of 

 observation, and need not be mixed with geometrical analysis ; 

 and we recommend the reader to refer to actual facts, and judge 

 for himself. 



CHAPTER XV. 



ON THE POSITIONS, UNDULATIONS, CONTORTIONS AND 

 FRACTURES OF THE SEDIMENTARY ROCKS. 



That disturbing forces have acted on the sedimentary rocks 

 during and after their formation, is abundantly proved by their 

 general appearance in all parts of the world. We have shown that 

 the surface of the dry land has been cleaved, fractured and dis- 

 located, and that there is scarcely an area of a few square miles 

 which does not bear marks of having been acted on by subter- 

 ranean forces. Since this is the case with the fundamental cry- 

 stalline base, it is reasonable to suppose that the sedimentary 

 beds deposited on it would be similarly affected ; not simply by 

 the periodical and sudden effects of earthquakes, but by the quiet 

 and insensible slow action of the magnetic currents, perpetually 

 altering the plane of the beds by numerous undulations and 

 different relative positions ; so that a series may commence to 

 be deposited on a concave, half formed on a level plane, and 

 completed on a convex surface ; sometimes receiving the elements 

 on the one side, and again on the other, according to the nature, 

 amount and continuity of the subterranean forces. 



Suppose, for instance, we find a series of rocks in the north- 



* Persons who have been led to suppose, by the established geometrical 

 theory of the ' Principia,' that at the equator centrifugal and centripetal forces 

 are equal, and that the trade-winds sweep over the zone with great force 

 always in one direction, &c, may probably be surprised when informed that 

 such is not the case. 



