THE TANGANYIKA PR OB LEA /. 27 



salt may crystallise out on the reeds and beaches in glitter- 

 ing white precipitates, as I have seen it repeatedly round 

 Shirwa and the other salt-pans of the African interior. A 

 lake having no outflow bears the same relation to its 

 drainage area that the ocean does to the whole land sur- 

 face of the earth, the volume of water in the sea remains 

 constant, but vast quantities of this water are for ever 

 evaporating into the atmosphere, and are then flung broad- 

 cast over the surface of the land, from whence they 

 eventually drain back in the form of rivers, but each in 

 possession of a certain quantity of salt which they add to 

 that already in solution in the sea. The sea is nowhere 

 nearly saturated with salt, and the above reasoning would 

 appear to lead to the conclusion that if we had a sample 

 of the ocean which was sufficiently old we should find that 

 it contained less salt than does the present-day sea-water ; 

 whether, however, we can agree with Hunt that the water 

 sometimes enclosed in ancient rocks may be looked upon 

 as fossil sea-water, and consequently that the ancient 

 oceans were richer in magnesium and potassium than the 

 present-day sea, is a matter into which we need not at 

 present go ; all that it is important to realise being, that 

 it seems to be demonstrable from different sources that 

 the sea has changed and is changing in composition with 

 a gradual increase of common salt in solution, and that the 

 indication of a physical change in the sea which we obtained 

 from the purely zoological considerations, discussed in the 

 preceding pages, seems to be in unison with various other 

 lines of evidence. 



There thus appears to be reason to suppose that there has 

 been a slow increase of common salt in solution in the sea, 

 and if this is so, it would also appear that the general 

 course of marine evolution may have been profoundly in- 



