38 A TREATISE ON METAMORPHISM. 



of the original textures of sedimentary rocks, such as granular and oolitic, 

 there may be produced textures characteristic of the metamorphic rocks, 

 such as cataclastic, parallel orientation, etc. The alteration may result in 

 the change to minerals all of which may wholly differ from any of the 

 original minerals, or it may take place by recrystallization without change 

 in mineral character, as in the case of the formation of marble from lime- 

 stone. Chemical change may result in the addition of constituents, as in 

 the case of oxidation and hydration of compounds already existing, or in 

 the deposition of additional material in the interstices, or in the abstraction 

 of material. Any given mineral may gain additional elements, or a greater 

 proportion of some of the elements; it may lose a part or all of some of its 

 elements, or it nia}^ be wholly replaced by another mineral. While the 

 chemical composition of the rock may be greatly affected by such changes, 

 in other cases the alterations may result merely in a redistribution of the 

 elements without affecting the average composition of the rock, as in the 

 case of marmorization, some cases of devitrification, various cases of 

 metasomatism, etc. 



After one set of changes has taken place, or while they are in prog v ess, 

 a change of physical conditions may come about in consequence of which 

 a different set of changes may be set up. Thus rocks may be partly modi- 

 fied under mass-static conditions and subsequently modified under mass- 

 mechanical conditions. They may be modified near the surface of the 

 earth, and as a result of burial be later modified at much greater depth; 

 or they may be modified at great depth, and as a result of erosion be 

 brought near the surface and there be again modified. Therefore one set 

 of changes may be superimposed upon another. In many cases it is cer- 

 tain that rocks have gone through several very complex sets of modifica- 

 tions. For instance, a rock may be modified under conditions at the 

 surface, afterwards be buried under other strata and thus pass into a deep 

 zone, where it may be modified in a different manner, and still later, as a 

 result of denudation, be brought to the surface and in the passage undergo 

 successive alterations in intermediate belts, and when it reaches the surface 

 once more be altered by the same forces and agents as at first. Substan- 

 tially this history has been gone through by the jaspilites of the Lake 

 Superior region. (See pp. 831-833.) Many other rocks have had an 

 equally intricate but very different history. 



