684 A TREATISE ON METAMOEPHISM. 



allanite, chondrodite, andalusite, sillimanite, cyanite, staurolite, tourmaline, 

 tlie micas, cliloritoid, and ottrelite; and of course various subordinate 

 silicates, such as axinite, melilite, gehlinite, vesuvianite, zircon, pied- 

 montite, topaz, etc. The only silicates which bear appreciable amounts of 

 water are the epidotes, staurolite, and the micas. The ordinary com- 

 pression conditions of the zone are not strong- enough to completely 

 dehydrate these minerals. It therefore appears that the most profound 

 conditions of metamorphism with which we are familiar are not sufficient 

 to drive off all of the water from all the minerals. 



It has been explained that the hydrated, carbonated, and highly 

 oxidized minerals produced in the zone of katamorphism may, when buried, 

 pass into the zone of anamorphism. It has also been explained on pages 

 366-369 that in these two zones the reactions are reversed. However, 

 it by no means follows that when a rock altered in the belt of weathering 

 or the belt of cementation has passed into the zone of anamorphism the 

 minerals originally present in the rock will be reproduced in their original 

 proportions. Indeed, it is certain that this will rarely, if ever, occur. The 

 reason for this is that in the zone of katamorphism, and especially in the 

 belt of weathering, there may have been great changes in the chemical 

 composition of the rocks. For instance, it has been pointed out that the 

 rocks are very much depleted in alkalies and alkaline earths in the belt of 

 weathering, and that in the belt of cementation the addition of silica is 

 very common. It is therefore clear that when rocks of changed chemical 

 composition pass into the lower zone the minerals which were originally 

 present before the rocks were altered in the zone of katamorphism may not 

 all be reproduced, nor will those there formed develop in the same propor- 

 tion as originally. Furthei'inore, the sediments are likely to be deficient in 

 certain elements as compared with original igneous rocks. One element 

 which is especially deficient in these is sodium. It naturally follows 

 that when a sedimentary rock is transferred to the zone of anamorphism 

 by burying, such soda minerals as leucite, nephelite, and sodalite are 

 rarely, if ever, produced, simply because of a deficiency of one of the 

 elements out of which they may be made. But even where unchanged 

 igneous rocks are recrystallized in the zone of anamorphism the minerals 

 are not preserved in the same proportion, for it has been explained that 

 pressure tends to produce molecules which are heavy, and consequently 



