the Revolutions which have taken place on the Globe. 243 



the clearest line of demarcation that can be found between 

 two systems of consecutive sedimentary deposits. This idea, 

 which has been developed in the lessons of the most distin- 

 guished professors, h^s, it may be said, become common. It 

 was indeed on a fact of this nature, generalized certainly be- 

 yond measure, that Werner founded his principal division in 

 the series of rocks. Now it follows from this difference, 

 always clear and without passage, between the upheaved beds 

 and those which are horizontal, that the elevation of the beds 

 has not been effected in a continuous and progressive manner, 

 but that it has been produced in a space of time comprised 

 between the periods of deposition of the two consecutive rocks, 

 and during which no regular series of beds was produced; — 

 in a word, that it was sudden, and of short duration. 



It has been in vain attempted to explain the geological facts 

 observable in high mountain-chains, by the action of the slow 

 and continuous causes now in force on the surface of the globe. 

 No satisfactory result has been obtained by these means. In 

 fact, everything shows that the instantaneous elevation of the 

 beds of a whole mountain-chain is an event of a different order 

 from those which we daily witness. It is evident that such a 

 convulsion would interrupt the slow and progressive forma- 

 tion of the sedimentary deposits, and that some anomalous 

 circumstance would be nearly universally observable in that 

 point of the series of rocks which should correspond with the 

 moment when an elevation of beds took place. It is well 

 known that those geologists who have most carefully exa- 

 mined the sedimentary deposits, and those naturalists who 

 have investigated the remains of animals and vegetables which 

 they contain, have generally remarked that between different 

 terms of the series of these rocks there are sudden variations, 

 not only in the position and local character of the beds, but 

 also in the fossil animals and vegetables entombed in them. 

 From observations which did not comprise a sufficiently ex- 

 tensive area, some of these variations (to lessen the value of 

 which too many attempts have perhaps been subsequently 

 made) were at first supposed more general than they really 

 are. When two formations appear to pass insensibly into 

 each other, there is never more than a small depth of beds 

 of which the classification may remain uncertain; and when 

 certain fossils are common to two successive formations, they 

 generally constitute a fraction, often even inconsiderable, of 

 the total number of species found in each of the two forma- 

 tions. This is more particularly seen in the comparison in- 

 stituted by M. Deshayes (in a work im|)atiently expected by 

 geologists) between tiic catalogues of tiie species of shells 

 2 I 2 discovered 



