

o 



O 



08 



CAUSES OF VARIATION 



[Ch. XIV. 



may mult 



ao-es and decompose, leaving no vestige of their form or sub- 



stance behind. 



more 



com 



Nor 



can they be perpetuated for indefinite periods in a fossil state, 

 unless imbedded in some matrix which is impervious to water, 

 or which at least does not allow a free percolation of that 



fluid, impre 



carbonic or other acid. 



may 



\ 



matrix 



mi 



mm 



matter 



peded the fossil shell or bone will be dissolved and removed, 

 particle after particle, and thus entirely effaced, unless petri- 

 faction or the substitution of * " JJ "~ 



happen to take place. 



That there has been land as well as sea at all former 

 geological periods, we know from the fact, that fossil trees 

 and terrestrial plants are imbedded in rocks of every age, 

 except those which are so ancient as to be very imperfectly 

 known to us. Occasionally lacustrine and fluviatile shells, 



am 



conclusion. The existence of dry land at all periods of the 

 past implies, as before mentioned, the partial deposition of 

 sediment, or its limitation to certain areas ; and the next 

 point to which I shall call the reader's attention, is the shift- 



om 



First, then, variations in the site of sedimentary deposition 

 are brought about independently of subterranean movements. 

 There is ° always a slight change from year to year, or from 

 century to century. The sediment of the Ehone, for example, 



own 



mile and a half distant from that where it accumulated in the 

 tenth century, and six miles from the point where the delta 



began originally to form. 



We 



forward to the period 



when this lake will be filled up, and then the distribution of 

 the transported matter will be suddenly altered, for the mud 

 and sand brought down from the Alps will thenceforth, in- 

 stead of being deposited near Geneva, be carried nearly 200 

 miles southwards, where the Ehone enters the Mediterranean. 









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