OBSERVATIONS ON SHELL-HEAPS AND SHELL-BEDS. 307 
is afforded by the fact of nearly all the streams and estuaries 
having bar entrances, which in some instances become entirely 
blocked up until a passage is opened by land-floods. Notable 
instances may be mentioned as those of Lake Macquarie, the 
Hastings, Nambucca, Bellinger, Brunswick, and Tweed Rivers ; 
the rivers mentioned, though thus impeded at the mouth, have 
bold water and long reaches further inland, and have apparently 
at one time had a much freer discharge. The same phenomenon 
in a more noticeable degree is to be seen in smaller streams now 
entirely blocked to ordinary tidal influence, and left, as it were, 
high and dry. In these latter, oysters and other marine shells no 
longer exist, though traces of their former abundance are evident. 
The elevation of the land has doubtless in no small degree con- 
tributed to the filling-up of the larger estuaries. For instance the 
Clarence, in which it is evident estuarine conditions have extended 
much further than they do at present, for at Lawrence, twenty- 
four miles from the sea, I have found shell-beds under alluvial 
deposit, at or below the level of the present high-water mark, 
and these beds are frequently come upon in the islands lower 
down the river, though the present zone of living shell-fish does 
not extend more than five miles from the sea in consequence of 
the unfavourable influence of the large volume of fresh-water 
brought down by the river; the effect of such excess of fresh- 
water during a prolonged freshet, is to entirely destroy these 
shell-fish, and in the case of oysters it is said to take from two to 
three years to re-establish the beds. On the Richmond River, 
the lower part of which is a delta formation, very similar to that 
of the Clarence, the existing oyster-beds are confined principally 
to the estuary of North Creek, and only extend in gradually 
diminishing quantity for about three miles up that creek, whilst 
the shell-heaps lately described by Mr. Darley are met with in 
exactly the reverse order as regards quantity, being more abundant 
up the creek, and are to be found far above the limit of present 
favourable conditions. 
There are also deposits of shell of considerable extent along 
Emigrant Creek, in which no living shell is to be found, and these 
