906 A TREATISE ON METAMORPHISM. 



be definite for a given rock, one can specify what alterations are likely to 

 occur, so far as each mineral is concerned, and also the alterations which 

 are likely to take place from a particular combination of minerals. It is 

 believed that this information will be of great service in a future systematic 

 study of the metamorphism of the various igneous rocks in the different 

 belts and zones, of metamorphism, for it is a great advantage to have definite 

 suggestions as to what may occur. Such suggestions in effect ask the 

 question, when one is making observations, whether or not the facts of 

 alteration for each rock in the different zones and belts correspond to the 

 expectation. If they do the suggestions are confirmed, if not, advance has 

 still been made upon the problem as to the physical conditions under which 

 the alterations observed actually took place. 



Once a fair degree of agreement is reached as to the orders, families, 

 and species of the original igneous rocks, it will be possible to trace out 

 their alterations in the various zones and belts in a manner somewhat similar 

 to that in which the alterations of the sedimentary rocks have been 

 imperfectly traced out. This, however, is plainly a work which will not 

 be accomplished for many years. In the meantime this chapter furnishes 

 guideboards telling how a great territory of petrography, only peered into 

 from the borders, may be occupied. 



