ORIGIN OF DOLOMITE. 801 



may be gathered together under a general principle, viz: On the average, 

 limestones in regions of strong orogenic movements, and consequent frac- 

 turing, are more strongly magnesian than limestones of equivalent age in 

 less disturbed regions. To illustrate, the Tertiary limestones of the Coast 

 Range of California and of the Alps are more strongly magnesian than 

 the undisturbed limestones of the same age. When the cases of variable 

 magnesium are brought together under this rule, the reason for the 

 variation is plain. Limestones are likely to be magnesian in proportion as 

 there were favorable conditions for the entrance of solutions bearing 

 magnesium, and these conditions are of course furnished by orogenic 

 movements and fractures causing openings. The variations in magnesium 

 content, rapid and slow, above cited, and other similar instances, can 

 apparently be explained only on the theory of secondary replacement. 



(4) Taking the world as a whole, the older a limestone formation is 

 the further advanced is dolomitization. Thus the ancient limestones, 

 especially those of Cambrian and Silurian age, are very generally strongly 

 magnesian, and the pre-Cambrian limestones are usually completely 

 dolomitized. This is partly a direct consequence of (3). For in propor- 

 tion as formations are old they are likely to have been subjected to orogenic 

 movement and to fracture. Also in proportion as they are old sufficient 

 time has elapsed for the other favorable conditions to have occurred. But 

 the principle of increase in dolomitization with age must be broadly applied, 

 for, as already seen, a young formation may be strongly dolomitic in one 

 region, and a much older formation be only slightly dolomitized in another 

 region; also, in various districts heavily magnesian formations occur above 

 limestones containing little magnesium. 



(5) It has been shown on page 239 that the change of calcite to 

 dolomite involves a contraction of 12.30 per cent. It is a well-known 

 fact that dolomites which have not been subjected to strong orogenic 

 movement, deep below the surface, are very porous. This is beautifully 

 illustrated by the dolomites of the Mississippi Valley, especially the 

 so-called magnesian limestone series. In this respect the dolomites contrast 

 strongly with the pure limestones. The natural explanation is that the 

 porosity is due to the contraction of the volume as a result of dolomitization. 

 Where the change has taken place in the zone of anamorphism, or where 

 the dolomite has been in the lower zone since the change, mashing and 

 recrystallization have closed the openings. 



MON T XL^'IT — 04 51 



