272 A NATURALIST OX THE "CHALLENGER?* 



bies swim across occasionally, and may add their bones to the 

 debris at the bottom. 



Hence here is being formed a sandy deposit, in which may 

 be found Cetacean, Marsupial, bird, fish, and insect remains, 

 together with land and sea-shells, and fragments of a vast land 

 flora ; yet how restricted is the area occupied by this deposit, 

 and how easily might surviving fragments of such a record be 

 missed by a future geological explorer ! The area occupied by 

 the deposit will be sinuous and ramified like that of an ancient 

 river-bed. 



The inlet being so extremely long and so narrow, although 

 the rise of the tide is two feet or more at the head of the creek, 

 the interchange of water with the ocean is very small; the 

 water in the upper parts of the creek, is merely forced back to 

 a higher level by the tide below at flood-tide, and similarly 

 lowered again at ebb. Hence, after heavy rain, the surface 

 water in all the upper parts of the creek is so diluted by the 

 torrent of fresh water from the stream, that it becomes almost 

 fresh ; indeed, at the time of our visit, it was for three or four 

 miles down, which was, as far as we went, so little brackish as 

 to be drinkable. At a short depth, no doubt, the water was salt. 



Here are the most favourable conditions possible for turning 

 marine animals into freshwater animals ; in fact the change of 

 mode of life presents no difficulty. Below, no doubt, the water 

 is always salt, but the fish find a fluid gradually less and less 

 salt as they rise to the surface. 



We caught the mullets in the almost fresh water, with a net. 

 The oysters were flourishing in the same water, and with them 

 the mussels and crabs ; I even saw an abundance of Medusa?, 

 and a species of Rliizoplwva swimming in the creek above the 

 sand-flats, where there was scarcely any salt at all in the water, 

 yet evidently in most perfect health. 



Occasionally, in times of long drought, the water becomes as 

 salt as the sea. The fishermen told me that after sudden very 

 heavy freshets of water from the river, some of the shell-fish 

 sickened and died. He accounted for the presence of numerous 

 dead cockle-shells (Cardium) in the bed of the creek, since 

 he had never found the animals there alive, by supposing that 



