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EVIDENCE OF THE RELATIVE AGE OF STRATA. 



The evidence from the superposition of strata, or what the French 

 call gisement, is based upon a self-evident truth. In all stratified 

 rocks that have been formed or deposited by water, the lowest stra- 

 tum is the most ancient ; or, in other words, every stratum is older 

 than the stratum that covers it ; unless, by some violent dislocation, 

 the strata have been overturned, or removed from their original posi- 

 tion. What is true with respect to two strata, may be applied to two 

 series of strata, that occur under each other : thus, we are certain 

 that the red sandstone and marl under the lias beds, are more an- 

 cient than the latter ; and as both formations preserve the same char- 

 acter over a great extent, whenever we meet with them in other sit- 

 uations, where the superposition is not apparent, we may safely con- 

 clude, that the red sandstone is more ancient than the lias, and oc- 

 curs underjt. 



We cannot, however, apply the same evidence to two groups of 

 strata formed in detached lakes or basins, because, being deposited 

 in different localities, they never occur superimposed on each other. 

 Let us suppose that two ancient lakes, situated at a considerable dis- 

 tance, had become dry in remote ages, and that a stratum of calca- 

 reous marl were found in the ancient bed of each lake ; it would be 

 evidently impossible, from these data, to determine which stratum 

 was the most recent, or whether their ages were coeval. Let us, 

 for the better distinction of the stratum of calcareous marl in each 

 lake, call the one stratum A, the other B. Suppose the geologist, 

 who had seen the marl beds, were to observe, in a neighbouring sleep 

 bank or cliff, two marl beds similar to A and B, but separated by a 

 bed of sandstone, he would have no doubt that the lower marl was 

 the most ancient ; but he could not apply this to determine the rela- 

 tive ages of the lake-marl strata, A and B. Were he, however, to 

 discover a number of shells of one species in the lower marl bed of 

 the cliff, and another species in the upper marl bed ; and were he af- 

 terwards to find the same species of shells that were in the lower 

 cliff marl, in the lake-marl bed A, and the species that were in the 

 upper cliff marl, in the lake marl bed B ; he would then have strong 

 presumptive evidence, that the lake-marl A was more ancient than 

 the lake marl B. The evidence from organic remains, or what is 

 technically called the zoological characters, becomes moie satisfacto- 

 ry in proportion to the number of instances in which it can be sup- 

 ported by the evidence from position. 



In the above example of the strata of calcareous marl in the two 

 ancient lake beds, the evidence of their relative ages derives all its 

 value from the original evidence of position observed in the marl beds 

 of the cliff. The evidence from organic remains alone, must ever 

 be attended with uncertainty, unless originally confirmed by the evi- 

 dence from superposition. Animals whose remains are deposited in 

 distant basins, may be of different species ; but this does not prove 

 that they did not live at the same period, as we find in the present 



