Hydrographical Observations from the Danmark Expedition. 387 



smell of hydrogen sulphide when taken up, as this hydrogen sulphide 

 must come from the reduction of sulphates. When I obtained the 

 samples, however, this smell had disappeared. 

 Copenhagen, February 1910. 



(Sign.) Niels Bjerrum. 



The result of the investigation is, that the water was certainly 

 sea-water. As it smelt strongly of hydrogen sulphide when taken 

 up, it must have been for some time at the bottom of the lake. 

 But how long? Had the land risen and thus converted a fjord into 

 an inland lake? 



This is indicated by a great resemblance of the gravel plain, 

 through which the river flowed, to a raised sea-bottom, the occurrence 

 of bivalve shells etc. Further, drift-wood was found along the bank 

 of the lake and a fossil skeleton of a whale on the southern bank. 



Closer consideration shows, however, that the salt water must 

 have arisen from a much later period than that, when the lake pos- 

 sibly was at a level with the sea. 



Diffusion between the salt bottom-layer and the fresh surface- 

 layer ■will, namely, constantly tend to reduce the salinity in the under 

 layer and this process goes on with a rapidity which is expressed in 

 the follo^\^ng formula: 



dc dc 



dS = — к — dt, where — = the difference per cm. in S p. m. 



dc . ^^ . ^^ . 



when — is greatest; t is the time in days, dS is the quantity of 



salt in gm., which passes through a section of 1 cm^ in unit of time. 



In Sælsø this will amount to 0.4 gm. in the course of a year. 



If we take the thickness of the salt layer to be on an average 

 60 m. and the salinity as 28 p. m., a cylinder of this height and of 

 1 cm^ in diameter wU contain 168 gm. of salt. 



If the diffusion process continued with unchanged rapidity of 



0.4 gm. salt through each cm^ per year, all the salt would thus be 



washed out of the bottom-water in the course of ca. 400 years. 



dc 



— will decrease, however, as the salinity decreases in the lower 



dx 



layer, whereas the upper layer will always remain nearly fresh, o\ving 

 to the downflow from the glacier, and will flow out through the river. 

 These calculations show, that the salt bottom-water cannot pos- 

 sibly be of the same age as the time when the lake was presumably 

 an arm of the sea and when the drift-wood and the whale's skeleton 

 were deposited there. The average salinity of the bottom-layer is 

 28 p. m. and if we take the average salinity of the sea-water out- 

 side to be 32 p. m., it can only have been ca. 50 years since 

 the sea-water filled the lake. 



25* 



