SIR CHARLES LYELL 209 



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 petrification or the sub- 

 stitution of some mineral for the organic matter 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 lacrustine and 

 fluviatile 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 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 shifting of these areas from one re- 

 gion to another. 



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 Rhone, for example, thrown in the Lake of 

 Geneva, is now conveyed to a spot a 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 may look for- 

 ward to the period when this lake will be filled up, and then the dis- 

 tribution of the transported matter will be suddenly altered, for the 

 mud and sand brought down from the Alps will thenceforth, instead 

 of being deposited near Geneva, be carried nearly 200 miles south- 

 wards, where the Rhone enters the Mediterranean. 



In the deltas of large rivers, such as those of the Ganges and Indus, 

 the mud is first carried down for many centuries through one arm, and 

 on this being stopped up it is discharged by another, and may then 

 enter the sea at a point 50 or 100 miles distant from its first receptacle. 

 The direction of marine currents is also liable to be changed by vari- 

 ous accidents, as by the heaping up of new sandbanks, or the wearing 

 away of cliffs and promontories. 



But, secondly, all these causes of fluctuation in the sedimentary 

 areas are entirely subordinate to those great upward or downward 



