42 G-EOLOGICAL AGE AND FORMATION. 



must, at least, equal the whole depth from high- water 

 mark to the lowest points at which stumps and roots of 

 trees have been found in their places of growth. This, 

 from the evidence on p. 34, is seventeen feet, and it may 

 be more. 



In this connection it may be interesting to again notice 

 the facts mentioned on pp. 27 and 28, of the existence of 

 shells of the same species now inhabiting the adjacent bay 

 and ocean, at elevations considerably above the present 

 tide-level. From this we may fairly infer, that the present 

 period of subsidence was preceded by one of elevation, in 

 which, what was before the bottom of the sea or bay, was 

 carried upward at least twenty-five or thirty feet above 

 high-water. And there may have been several alternate 

 periods of elevation and depression; of which that, when the 

 timber and shells of the Alluvium were buried, must have 

 been one of depression. 



In tJiese alternations of elevation and subsidence we may 

 find a reasonable explanation of the various phenomena 

 connected with the upland Alluvium. If we go back to 

 the commencement of the period of subsidence preceding 

 the present, for our starting point, and assume, as there is 

 reason to, that the ground was a little higher than it now 

 is, then, as the ground slowly sunk down, the water would 

 overflow the upland, killing the timber, and carrying it 

 beneath the level of the tide. This subsidence must have 

 continued until most, if not all, of the present upland of the 

 county was below tide-level. Oysters and other shell-fish 

 would of course be found wherever the sea-water extended. 

 The action of the waves would wash out the loam and finer 



