166 



GEOLOGY. 



[Sect. VI. 



It is liighly necessary most carefully to keep the fossils 

 found in different strata separate ; it will often occur iu 



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passing upwards from one bed to another, and occasion- 

 ally even without any great change in the character of 

 the rockj that the fossils will be wholly different ; and if 

 such distinct sets of fossils are mingled together, as if 

 found together J undoubtedly it would have been better 

 for the progress of science that they had never been 

 collected. As there is some inconvenience in keepino* 

 the fossils collected on the same day separate, this caution 

 is the more requisite. The collector, if he be not an 

 experienced naturalist, should be very cautious in reject- 

 ing specimens, from thinking them the same with what 

 he has already got ; for it requires years of practice to 

 perceive at once the small, but constant, distinction 

 which often separate species: the same species, moreover, 

 if collected in different localities, or in bt ' ; one placed 

 far above the other, are generally more valuable to the 

 geologist than new species. 



In formations from a few hundred to a thousand feet 

 and upwards in thickness, the whole of which doe 



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3 



actually belong to the same geological age, and is there- 



fore characterized by the same fossils, most curious and 

 important resiiUs may be sometimes deduced, if the posi- 

 tion or relative heights at which the groups of fossils are 

 embedded be noted ; and this is a point usually neglected. 

 For, thanks to the researches of Professor E. Forbes, the 

 depth of water under which a collection of shells lived 

 can now be approximately told ; and thus the movement 



of the crust of the earth, whilst the strata 



iucluding Lie 



shells were accumulating, can be inferred. For iiustance, 



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