WOKK IN BELT OF CEMENTATION. 629 



CAUSES OF CEMENTATION. 



It has already been explained (see p. 156) that the amount of ground 

 water which reaches the surface by springs and by seepage for a given 

 period is approximately equal to the amount which is added to the sea of 

 ground water. It is well known that all issuing waters contain material in 

 solution; and it has been shown that the amount of such material may 

 be greater than that which the solutions contained when they entered the 

 belt of cementation. Notwithstanding the foregoing conclusion, it is an 

 undeniable fact, as fully shown in the previous pages, that cementation is 

 the rule for the part of the zone of fracture below the level of ground 

 water. If more material be dissolved in this belt than is deposited by the 

 circulating waters, what is the explanation of the apparently contradictory 

 fact of deposition to the extent of complete cementation? 



Before attempting to answer this question, it is advisable to recall the 

 amount of openings which may be present in the belt of cementation. It 

 has been shown that the coarse mechanical sediments frequently have an 

 original average pore space as great as one-third; that in the thinly bedded 

 lavas this amount is often equaled, and that the openings produced by 

 mechanical action are great. It follows, as already clearly shown, that the 

 amount of deposited cementing material is vast. Therefore we must not 

 only furnish a cause which will result in the deposition of material in the 

 pore spaces, but we must show that the causes assigned are quantitatively 

 adequate to perform the great work. 



In this connection it is well to restate the principles of precipitation 

 worked out on pages 113-123. Mentioned in order of probable impor- 

 tance, precipitation is due (1) to the mingling of solutions from different 

 sources, (2) to reactions between the solutions and the wall rocks, (3) to 

 decrease in temperature, and (4) to decrease in pressure. The compounds 

 are precipitated from the solutions in proportion as they are relativelv 

 insoluble and in proportion as they are abundant. 



The law of chemistry that when compounds of different kinds come 

 together substances form (if possible) which are insoluble in the liquids 

 present, explains the importance of the first and second causes of precipita- 

 tion. It has been shown that decrease in temperature is also very important, 

 and that probably the importance of this factor in connection with decrease 



