REVERSIBLE REACTIONS. 367 



the zeolites alter into the various feldspars. The above reaction chances 

 to be the only one given for these groups which is exactly reversed. This 

 is a consequence of the fact that reactions are written only for recorded 

 alterations. There can be no doubt that practically all the equations 

 representing the recorded alterations of the feldspars (pp. 261-263) into 

 the zeolites, and all the reactions representing the recorded alterations of 

 the zeolites (pp. 333-334) into the feldspars, are reversible. For instance, 

 we have anorthite altering into gismondite as follows (p. 262): 



3CaAl 2 Si 2 8 +12H 2 0=Ca 3 Al 6 Si 6 24 .12H,0+k. 



Can one doubt that if gismondite passes into the zone of anamorphism 

 dehydration may take place and anorthite be reproduced! 



Another line of evidence pointing to the reversibility of the reactions 

 in the two zones is the frequent recorded association of corundum with 

 diaspore and gibbsiie, the latter minerals being secondary to the corundum. 

 Can it be doubted that these hydrates may be dehydrated in the zone of 

 anamorphism and reproduce corundum? Of course this particular change 

 may not occur alone. At the same time the dehydration takes place the 

 alumina may unite with silica and form andalusite, sillimanite, or cyanite, 

 or the alumina may enter into some other silicate. 



Bearing' in the same direction are the experiments made by Daubre'e 

 upon serpentine. a It is well known that both enstatite and olivine alter 

 into serpentine. Daubre'e found that by the fusion of serpentine it split up 

 into enstatite and olivine, according to the following equation: 



H 4 ilg 2 Si 2 9+ Heat=MgSi0 3 +.AIgSi0 4 +2H 2 0+k. 



Finally, my chief reason, in addition to those already given, for belief 

 in the reversibility of the reactions in the two zones lies in the actual 

 compositions of the unmetamorphosed sediments and their metamorphosed 

 equivalents. The unmetamorphosed pelites are composed largely of the 

 lighter hydrous minerals of the belt of weathering and the belt of cementa- 

 tion. It is true that with these, as already explained, there are also 

 considerable, or even dominant, quantities of residual undecomposed 

 anhydrous minerals; but it is certain that the metamorphosed equivalents 

 of these pelites contain none of the minerals which are characteristic of 



«Daubr£e, A., Experiences synthetiques relatives aux meteorites: Comptes rendus des seances 

 de l'academie des sciences, vol. 62, Paris, 1866, p. 661. 



