MOVEMENTS OF STRATA. i9 



deposited at the bottom of waters, they must have a nearly hori- 

 zontal position (fig. 1), and that they must occupy, the steepest 

 parts of the surface upon which they are formed, so that if the 

 surface presents considerable elevations, these may remain un- 

 covered, and show themselves above the level occupied by the 

 new formation (fig' 2). Thus when we go from low plain 



Fig. 2. 



towards mountain chains, and ascend to their summits, we meet, 

 successively, formations more and more ancient as we rise. 



32. Sometimes these stratified rocks preserve the horizontal 

 position they had in the beginning ; but at other times they become 

 more or less oblique in consequence of their partial depression or 

 sinking, or their unequal elevation. Frequently we see beds 

 which are abruptly raised up, so as to be almost perpendicular ; 

 and on the edges of the elevation produced by this overturning of 

 nature, we find other beds which are perfectly horizontal, and we 

 may conclude that the latter were formed subsequently to the eie- 

 vation of the former ; by studying these relations of position we 

 are enabled to determine the geological age of mountains. 



33. These great movements of strata sometimes take place sud 

 denly, and are accompanied by earthquakes ; but at other timrs 

 they are effected gradually and without any shock. It appears to 

 be well ascertained that since the time of the Romans, a portion 

 of the coast of Naples sank below the level of the sea, and was 

 subsequently raised up again above this level, witiiout overturning 

 the monuments built on this movable soil. One may be satisfied 

 of this fact by visiting an ancient temple situated near Puzzuoli, 

 called the Temple of Jupiter Serapis ; this monument, of which 

 three cohimns remain standing erect, appears to have been built in 

 the third century, and was then very much frequented, on account 

 of its warm baths ; but at a subsequent epoch, supposed to be about 

 1488, the ground sank down, and the temple was covered by the 



32. Do stratified rocks always preserve their original position ? What is 

 to be learned by studying the position of strata ? 



33. How do these great movements of strata take place? G re an in- 

 itance of the gradual movement of strata. 



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