Uniformity in Geological Change 



ment is accumulating, seaweed, zoophytes, fish, 

 and even shells, may multiply for ages and de- 

 compose, leaving no vestige of their form or sub- 

 stance behind. Their decay, in water, although 

 more slow, is as certain and eventually as com- 

 plete as in the open air. Nor can they be per- 

 petuated for indefinite periods in a fossil state, 

 unless imbedded in some matrix which is im- 

 pervious to water, or which at least does not 

 allow a free percolation of that fluid, impreg- 

 nated, as it usually is, with a slight quantity of 

 carbonic or other acid. Such a free percolation 

 may be prevented either by the mineral nature 

 of the matrix itself, or by the superposition of an 

 impermeable stratum; but if unimpeded, the 

 fossil shell or bone will be dissolved and removed, 

 particle after particle, and thus entirely effaced, 

 unless petrifaction or the substitution of some 

 mineral for the organic matter happens to take 

 place. 



That there has been land as well as sea at all 

 former geological periods, we know from the fact 

 that fossils trees and terrestrial plants are im- 

 bedded in rocks of every age, except those which 

 are so ancient as to be very imperfectly known 

 to us. Occasionally lake and river shells, or the 

 bones of amphibious or land reptiles, point to 

 the same conclusion. The existence of dry land 

 at all periods of the past implies, as before men- 

 tioned, 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 

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