244 A SHELL-BEAEING GEAVEL AT DUMBALL ISLAND 
although such forms as Tellina haltliica might perhaps 
have been able to live where they are now found. 
This latter view I should not care to emphasize were it 
not for the great number of specimens found throughout 
the shell-bearing deposit. Against this view it must be 
borne in mind that the large flat valves of Tellina are so 
admirably fitted for easy removal by moving water that 
they might well be carried to the spot where they 
are now found from deeper water outside. 
Reasoning from analogy, it is possible that older deposits 
of mud and fine gravel might yield similar land and fresh- 
water forms. Such species may even be known to us, 
but regarded as marine or brackish- water forms. At 
any rate, at Dumball Island we have evidence of the 
formation of an estuarine deposit, in which the majority 
of forms have been water-borne from land and freshwater 
areas. 
The deposits at Dumball Island have an interest of their 
own apart from the shell-bearing gravel. By reference to 
the section, it will be noted that the lowest bed of all is a 
bed of “ ballast ” ten feet thick. This “ ballast ” bed is 
really a bed of clean, large, well-rolled pebbles mixed with 
sand. It has all the characters of a pebble beach. Its 
presence at this depth and in this situation would seem to 
indicate either that the flow of the river Avon was so great 
at one period that pebbles could be carried or rolled as far 
down as Dumball, or else that the river bed was shallow, 
and its course farther out into the channel, so that all the 
mud carried in suspension was deposited outside the area 
in which the “ ballast ” occurs. 
