82 A. BLYTT. [NO. % 



more subject to earthquakes than others, we must also suppose 

 that was the case in Tertiary times too. If we disregard the Ter- 

 tiary beds of the great folded chains and neighbouring regions, 

 where great local upheavals and subsidences take place in rela- 

 tively short time, we find, that the Tertiary layers are mostly 

 fresh- water, littoral or sublittoral beds. An alternation of such se- 

 diments points to only slight displacements of the beach-line. 

 Edward Forbes, in his Eeport on the Mollusca and Kadiata of 

 the .Egean Sea, 1 says: „A very slight depression of land in the 

 Gulf of Macri on the coast of Lycia, would now plunge below 

 the sea muddy tracts abounding in Melania, Melanopsis, Neri- 

 tina and other fresh-water [Mollusca. JTheir successors in the 

 first formed shallows would be Cerithium mamillatum and a 

 few bivalves. A drift of sand over this Cerithium mud would 

 call into existence a new fauna and every successive depression 

 or elevation, however slight, would produce considerable zoolo- 

 gical changes, for the subdivisions of the uppermost region are 

 of small extent in depth, 2 and very liable to be affected by 

 secondary influences." How very slight these displacements were 

 in many cases, we learn from the fact, that in the same estuary 

 the same stage is, in some places, built through its whole vertical 

 extent only of fresh-water beds, whilst in other places marine layers 

 are intercalated in the middle, and that the relative thickness 

 of fresh-water and marine layers may vary in different loca- 

 ities. We have several such instances in the fluvio-marine ser- 

 ies of the Isle of Wight (Lower and Middle Headon etc). We 

 thus see, that the displacement of the beach-line was in such 

 cases so slight, as only to affect some parts of the estuary and 

 not the whole. 



I have, above, (p. 27-28) already pointed out, that, if » 

 deep basin is cut off f rom the ocean by embankments, a slight 

 shift of the beach-line may then be sufficient to cover or 



drain the bank, so that 



the basin may be alternately salt or 



