806 A TREATISE ON METAMORPHISM. 



part of the calcium of the solid be replaced by the magnesium. In a 

 limestone which has been slightly dolomitizetl the conditions are particularly 

 favorable for dolomitization, for under such circumstances under chemical 

 laws substitution goes on rapidly. But the further fhe process continues 

 and the nearer equilibrium is approached the slower it becomes. Since, on 

 the whole, calcium is a more abundant base in the sedimentary formations, 

 and also in the ground solutions, than magnesium, and since it is a more 

 energetic base, it follows that at low pressure one could not expect that 

 one-half of the calcium would be replaced by the magnesium, for when the 

 substitution had gone to a certain stage calcium would replace the deposited 

 magnesium as fast as the magnesium replaced the calcium. This stage 

 would usually be reached before the magnesium was molecularly equal 

 to the calcium. In the foregoing we have the explanation of the fact that 

 but verv rarely is the magnesium so abundant in extensive formations as to 

 have the precise composition of dolomite. 



In determining- the stage at which the static condition of affairs is 

 reached pressure is very important. It has been seen that as a result of 

 the substitution of magnesium for one-half the calcium in calcium car- 

 bonate there is a contraction in volume of 12.30 per cent; hence pressure 

 tends to promote the change. But pressure increases with depth; therefore 

 the deeper a rock is, the greater is the tendency for magnesium to replace 

 the calcium. Hence in the deeper buried rock, there is a strong force which 

 tends to substitute magnesium for calcium. But the complete substitution 

 is resisted, under the law of mass action, by the greater abundance of 

 calcium, and by the more energetic character of that base. Further, as 

 alreadv pointed out, where the pressure is very great, as in the zone of 

 anamorphism, circulation is very slow, and it is not possible for the solu- 

 tions to supply sufficient magnesium to accomplish this, and probably in 

 most cases not even enough to change the rock to dolomite. Usually these 

 factors prevent the substitution from going so far as to produce equimo- 

 lecular quantities of the two elements; but in the case of the pure dolomites 

 this occurs. In such instances it may be that pressure is an important 

 factor. Since few carbonates contain more than an equimolecular amount 

 of magnesium as compared with the calcium, it may be supposed that this 

 compound gives the limiting effect of such pressures as have obtained in the 

 rocks which are within the zone of observation. As a corollary from the 



