EFFECT OF QUANTITY OF ELEMENTS. 95 



Although, as just seen, strong bases and acids have a great advantage 

 over weak bases and acids, the quantity of an element present is a very 

 important factor in the final result of the action of the solutions on the solids. 

 If a certain element is abundant in the ground solutions, it may to a large 

 extent replace another element in the solids, an element of the solid going 

 into solution at the same time. This may take place to a large extent even 

 if the element in solution is weaker than the one it replaces in the solid. 

 For instance, the relatively weak base, magnesium, when abundant in 

 solutions, is known to replace the stronger base, calcium, on a large scale 

 in calcium carbonate, thus changing limestone to dolomite. In this reaction, 

 while the abundance of magnesium is a very important factor, a number of 

 others enter; and therefore its detailed consideration is given under the 

 process of rock dolomitization. (See pp. 802-808.) 



MECHANICAL ACTIOS. 



It has already been seen that no changes in rocks take place without 

 movements of material, small or great, for long or short distances. Even 

 in the case of a mineral passing from one form to an allotropic form, there 

 is movement of the molecules. In short, wherever there is rearrangement 

 of the elements there must be movements. 



Mechanical action alone is one of the processes of metamorphism of 

 the utmost importance. (See pp. 46-50.) However, the effect of mechan- 

 ical action in the promotion of chemical action is even more important 

 than mechanical action alone. 



Mechanical action influences chemical action in two general ways — the 

 speed is promoted, and the nature of the reaction is modified. 



SPEED OF CHEMICAL ACTION. 



The speed of chemical action is promoted directly by the deformation, 

 and indirectly by the heat liberated. 



DIRECT DEFORMATION EFFECT. 



As already shown (pp. 49-50), mechanical action produces deformation 

 in three different ways — by producing strain without rupture, strain with 

 rupture, and readjustment of the particles 



strain without rupture. — When material is strained without rupture, even if 

 the amount of deformation be slight, a great change in the molecular con- 

 stitution may be involved. This is well shown by a common experiment 



