14 F. L. Ekman, 



move with a greater velocity towards the lower latitudes, than towards 

 the parts of the evaporating region more distant from the equator. 



I do not know that attention has been paid to the importance of 

 evaporation for the formation of the cold under-current in the ocean '), 

 whereas evaporation has long been recognized as the cause of the cur- 

 rents, that exist between the ocean and separate basins such as the Me- 

 diterranean, in which the annual evaporation exceeds the rainfall. Even 

 if evaporation in such a basin were not more rapid than in the neigh- 

 bouring portions of the ocean, the level of the basin must nevertheless 

 sink relatively to the ocean's, because the contraction of the basin's 

 mouth in a certain degree obstructs the under-circulation, which in the 

 ocean carries water from localities of overabundant rainfall to those of 

 overabundant evaporation. As regards the nature of the stream, produ- 

 ced by evaporation, when passing a relatively narrow canal, such as 

 the Straits of Gibraltar, it must necessarily be very different from the 

 stream, which evaporation causes in the open ocean; for the pro^oortion 

 between the evaporating surface and the area of the section of the 

 stream is in this case altogether different. Should the section of the 

 canal in proportion to the quantity of water, that must pass through it 

 in a unit of time in order to compensate the evaporation, be, less than 

 a certain limit, there can be only one stream, which will flow in the 

 evaporating region, where the saltness in such cases must continuall}- 

 increase ^). When the depth of the section is greater, there may on the 



') lu tliis explanation I liave taken account of no other cause than evaporation, 

 nor of tlic circumstance, tliat by evaporation the salt is left in the water, whereby the 

 current might in time be considerably modified, even if exclusively arising from 

 evaporation. I shall have occasion further on to return to this question. 



-) Wc do not at present know of any such case ; but that under past geolo- 

 gical periods basins have been gradually so cut off from communication with the 

 open sea, that an effective double circidation became impossible, is evidenced by the 

 numerous deposited layers of ordinary salt, more or less visibly accompanied by other 

 ingredients of oceanic salt, as also by the pure sulphur in certain sedimentary forma- 

 tions, probably originating in sea-waters sulphates after their reduction through orga- 

 nic media. The process of these evaporating-operations has probably been, that when 

 the outward under-current, in consequence of the increasing shallowness of the mouth, 

 ceased, the contents of the basin have by the continued stream of salt water from 

 the sea been converted into a salt lake, which eventually christalized. It has then 

 depended on other circumstances, e. g. the affllu.x of river water, whether the residual 

 lye has been carried off, or as in the formation of the Stassfurter-strata, it lias dried 

 up. The new proposal to convert the Zahara-dcsert into a inland-sea would doubtless 

 lead to a similar result. 



