804 A TREATISE ON METAMORPHISM. 



given, that dolomitization below the sea is usually very partial, and that 

 the Metia instance is exceptional. It has already been seen that Forch- 

 hamraer found that late limestone deposits mainly composed of the remains 

 of animals usually contain less than 1 per cent of magnesium," and that 

 a small content of magnesium is the ordinary thing for late limestones 

 which have not been greatly disturbed by orogenic movements. In view 

 of these facts the question naturally arises as to the time when the dolomiti- 

 zation mainly took place in those extensive dolomite formations in which the 

 magnesium is moderately uniform for a given district, although slowly vary- 

 ing as a whole. The typical formations of this kind in America are the great 

 Cambro-Silurian limestones of the United States, which in the Appalachian 

 region, where deformation is great, are nearly pure dolomites, and which in 

 the Mississippi Valley are often heavily magnesian but rarely dolomite, 

 and show, moreover, rather sharp variations in the amount of magnesium.. 

 The hypothesis which one naturally favors, because of its simplicity, is that 

 such formations as these were mainly dolomitized below the sea. But the 

 more closely the facts are examined the less certain does the conclusion 

 appear to be. It is entirely possible that locally the Cambro-Silurian lime- 

 stone was dolomitized more extensively below the sea than is the case, on the 

 average, for the later limestone formations. Indeed, it has been supposed 

 that this limestone was deposited in a mediterranean sea, and this entire sea 

 may have had to some extent the concentrated conditions at Metia described 

 by Dana. But it appears certain to me, even if the dolomitization was 

 further advanced in the case of some of these formations while below the 

 sea than can be paralleled by recent extensive formations, that they have 

 subsequently been much further dolomitized and the magnesium extensively 

 rearranged since the limestones emerged from the sea. 



Dolomitization after limestone emerges from the sea. While it is dear that the pi'OCeSS 



of dolomitization may take place below the sea, often it is there only 

 initiated. Frequently dolomitization takes place in the main after the lime- 

 stone becomes a part of the land area and is subject to the forces of meta- 

 morphism beneath the air. It is believed that dolomitization in land areas 

 largely occurs in the zone of katamorphism rather than in the zone of 

 anamorphism. The basis of this belief is furnished by the conclusion, fully 



"Forchhammer, Georg, Bidrag til Dolomitens-dannelshistorie: Oversigt over der Kongelige Danske 

 Videnskab., Forhandlingar, Copenhagen, 18-49, p. 89. See also, Bischof, Gustav, Chemical and physi- 

 cal geology, London, 1855, vol. 2, pp. 48-49. 



