GEOLOGICAL CHRONOLOGY. 



the oldest being the lowest strata of the secondary, and the most 

 modern the upper strata of the tertiary ; the intermediate strata 

 have intermediate dates in the order of their superposition. 



Thirdly. The sedimentary strata in their original and natural 

 position were necessarily level; their hounding surfaces were 

 horizontal and parallel. Wherever, then, they may be found in 

 any other position, it must be assumed that they have undergone 

 derangement of position by some disturbing cause since their 

 original deposition. This derangement may arise either from the 

 strata being heaved upwards by a pressure from below, or by 

 their sinking downwards by their incumbent weight forcing them 

 into some inferior vacuity. 



Fourthly. Although the succession of strata constituting the 

 series, from the primitive rocks upwards to the surface, is by no 

 means invariable, and is subject on the contrary to many local 

 variations, still its general character is such as has been described. 

 If the sedimentary strata, proceeding from the lowest upwards, 

 when complete, be expressed by the letters A, B, c, D, &c., it will 

 sometimes happen, from local causes, that the actual series may 

 be A, B, c, E, &c., or A, B, D, E, &c., or A, D, E, &c., but it can 

 never happen that the series shall be D, B, A, c, &c. In a word, 

 one or more strata of the series may be wanting, but their natural 

 order is never inverted. 



44. It appears, therefore, that the character and order of the 

 sedimentary strata constitute a chronological scale indicative of 

 the history of their formation. It is true that the value of the 

 unit of this scale is not and cannot be known, inasmuch as the 

 absolute intervals of time, necessary for the deposition of the 

 strata severally, cannot be certainly determined. This, however, 

 does not prevent the geologist from pronouncing with perfect cer- 

 tainty upon the order of time of their deposition respectively. 



45. Although the series of strata above described have been 

 deposited, subject to so many local disturbing causes, that there 

 is probably no point on the entire surface of the globe, where a 

 section would exhibit them complete, still by a careful and 

 judicious comparison of observations, made in different localities, 

 their normal arrangement and natural order of superposition 

 has be*en ascertained, and geologists have grouped and classified 

 them under a great variety of denominations. Owing to the 

 absence of any general convention, no single system of nomen- 

 clature has been adopted, as has been so happily effected in 

 chemistry, and though in a less degree, also in zoology. The 

 consequence is that geology is overlaid with a complicated, con- 

 fused, and discordant nomenclature, detrimental to the diffusion 

 and even to the progress and extension of the science. Those 



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