41 



take place in the open sea. In this case, again, a knowledge of 

 what takes places in inland lakes enables us to gain some insight 

 into the origin of these deposits also. AVhen rain falls we all know 

 that some part of it flows over the surface to the nearest stream, 

 and thence away out to sea ; another part of it is evaporated ; and 

 the remainder sinks into the rocks, and soaks downward, until it is 

 stopped by an impervious or waterproof bed, whicli keeps tlie water 

 at that level until it finds its vi'ay out at the surface in the form of 

 a spring. In passing through the rocks the carbonic acid in the 

 rain-water enables it to dissolve various small quantities of the 

 mineral matter it is passing through, and to hold these in solution. 

 The Eden before us is at this moment carrying out to sea consider- 

 able quantities of such mineral matter, which it has collected from 

 the rocks in the upper part of its course. It is this mineral, 

 matter that makes, the water what we call "hard." When the 

 water of the spring finds its way through the rivers out to sea, this 

 mineral matter is lost sight of, at any rate for a time. But where 

 the water is discharged into a lake with no outflow, all the drainage 

 of the hydrographical basin around has to be got rid of by evapor- 

 ation. When impure water is evaporated the water only is carried 

 off, leaving the mineral matter behind. The consequence is that 

 the constant evaporation of the spring-water carried into these 

 inland lakes leaves year by year a gradually-increasing percentage 

 of mineral matter behind. After a time the percentage of salts 

 rises above the point of saturation — the water of the lake can hold 

 no more in solution — and then the various salts begin to be 

 precipitated in a solid form. The nature of these salts varies 

 according to the composition of the rocks traversed by the feeders 

 of the lake. In one instance it is carbonate of lime, in another 

 sulphate of lime, in a third carbonate of magnesia, in a fourth 

 carbonate of iron, that forms the principal mineral held in solution. 

 But even where several different salts are being brought down 

 concurrently, they are not all deposited at the same place, or at 

 the same time, because the point of saturation varies for each 

 mineral, and the deposition of one salt out of a complex solution 



