i88 THE BOOK OF NATURE STUDY 



out on a valley side or at a sea cliff, and especially if the strata 

 are inclined towards the low ground, there is a great tendency for 

 the upper pervious rock to slide forward on the unstable junction 

 between the two, and a landslip is thus formed. This causes 

 serious . damage and destruction to rocks on the surface of the 

 ground, and it passes down to lower levels great quantities of 

 rock in a broken-up condition, and ready to be dealt with by 

 the stream in the valley or the sea at the foot of the cliff (Fig. 75). 

 Whether the springs drain to the sea or to rivers, a vast amount 

 of solid matter is taken away by them in solution from the land, 

 and the height of the land is lowered eventually by the amount 

 thus chemically removed. To this must be added dissolved matter 

 taken up and carried by surface water. Some of the dissolved 

 constituents, like common salt, remain in solution in the sea 

 water, and appear to be slowly but steadily accumulating in 

 amount therein. But other constituents, especially carbonate, 

 sulphate and phosphate of lime, are made use of by animals, 

 like shellfish, corals, foraminifera, and fishes, and to some extent 

 by plants, in building up their shells, tests, or skeletons. At the 

 death of these organisms the materials are returned again to the 

 earth-crust on the sea bed, in the form of deposits of carbonate 

 or phosphate of lime, ready to make limestone rocks and phos- 

 phatic deposits. If the water does not reach the open sea, but 

 finds its way to inland lakes instead of the sea, where the only outlet 

 for water is by evaporation, the pure water alone is carried off, 

 and its dissolved salts are precipitated on the lake bed as deposits 

 of carbonate and sulphate of lime or magnesia, or as deposits of 

 rock salt. Some of the limestones, especially the magnesian 

 limestones and the chief gypsums and rock salts, found as con- 

 stituents of the earth-crust, have doubtless had this origin. 



