CHAP. \ii. CHANGES OF LEVEL. Ill 



through the underlying peat. Allowing about fourteen cen 

 turies for the growth of the superincumbent vegetable matter, 

 he calculated that the thickness gained in a hundred years 

 would be no more than three French centimetres.* This rate 

 of increase would demand so many tens of thousands of years 

 for the formation of the entire thickness of thirty feet, that 

 we must hesitate before adopting it as a chronometric scale. 

 Yet, by multiplying observations of this kind, and bringing 

 one to bear upon and check another, we may eventually suc 

 ceed in obtaining data for estimating the age of the peaty 

 deposit. 



The rate of increase in Denmark may not be applicable to 

 France ; because differences in the humidity of the climate, 

 or in the intensity and duration of summer's heat and winter's 

 cold, as well as diversity in the species of plants which most 

 abound, would cause the peat to grow more or less rapidly, 

 not only when we compare two distinct countries in Europe, 

 but the same country at two successive periods. 



I have already alluded to some facts which favour the idea 

 that there has been a change of level on the coast since the 

 peat began to grow. This conclusion seems confirmed by the 

 mere thickness of peat at Abbeville, and the occurrence of 

 alder and hazel-wood near the bottom of it. If thirty feet 

 of peat were now removed, the sea would flow up and fill the 

 valley for miles above Abbeville. Yet this vegetable matter 

 is all of submarine or fresh-water origin, for where aquatic 

 shells occur in it they are all of terrestrial or fluviatile kinds, 

 so that it must have grown above the sea-level when the 

 land was more elevated than now. We have already seen 

 what changes in the relative level of sea and land have oc 

 curred in Scotland subsequently to the time of the Eomans, 

 and are therefore prepared to meet with proofs of similar 

 movements in Picardy. In that country they have probably 



* Antiquites Celtiques, vol. ii. p. 134. 



