230 



PHYSICAL GEOLOGY 



of 21 feet above the base. At this time the Lithodomi bored into 

 the stone and made their homes there. The lower 12 feet of the 



columns were buried in sedi- 

 ment and therefore escaped 

 damage. Later the land was 

 again raised, and the columns 

 are now some distance from 

 the shore (Fig. 224). Such 

 evidence, although interest- 

 ing as showing recent changes 

 in sea level, is of minor im- 

 portance as compared with 

 the occurrence of strata con- 

 taining marine shells at 

 heights of 14,000 feet or 

 more above the sea. 



The Stability of the Atlantic Coast 

 of North America. — The statement 

 often made that the coasts of Nova 

 Scotia, New England, and New Jer- 

 sey have recently undergone a gradual 

 subsidence and that this movement 

 is still in progress, rests upon the 

 following evidence. 1 Stumps of 



Fig. 224. — Three columns of the temple of 

 Jupiter Serapis near Naples. The dark and 

 rough band above the figure is the portion 

 which was perforated by boring mollusks. The 

 lower portion of the columns was protected by 

 mud and the upper portion projected above trees are found in salt marshes ; salt 

 the sea. water is found overlying fresh-water 



peat ; marshes have increased in size ; 

 dikes erected to keep out the tide are themselves covered at high tide; a bench 

 mark at Boston is now three-fourths of a foot nearer the mean level of the sea than 

 when it was placed there three quarters of a century ago. When each case is care- 

 fully studied it is found either that the apparent sinking is due to local causes, or 

 that no definite conclusions can be drawn. The evidence from marshes is especially 

 uncertain, because when drained they settle; when sand dunes encroach upon them, 

 they are compacted and their surface is consequently lowered; when a bar behind 

 which fresh-water marshes and forests exist is cut through by waves (Fig. 225), the 

 marsh will be invaded by sea water, the trees will be killed, and salt-water peat may 

 in time cover the fresh-water peat. Changes in the direction or velocity of ocean 

 currents may also bring about local differences in sea level. The apparent lowering 

 of the bench mark near Boston is doubtless due to the narrowing of the bay as a result 

 of the artificial filling in of the marshes. Such a constriction of the channel would 



1 For a more complete statement see: D.W.Johnson, Science, Vol. 32, 1910, pp. 721-723, 

 and Fixitt de la Cote Atlantique de V Amerique du Nord, Annales de Geographie, Vol. 21, 

 1912, pp. 195-212. 



