MINERALS. 207 



SECTION 3.— ROCK-MAKING MINERALS. 



MANNER OF TREATMENT. 

 GENERAL STATEMENTS. 



In this treatise only the principal rock-forming minerals will be consid- 

 ered. The point of view is not that of mineralogy but that of metamorphism. 

 So far as it seems advisable without too much repetition, I shall consider 

 each mineral in reference to the following: 



(1) Its composition, crystallization, specific gravity, and source, so far 

 as it is a rock-making mineral ; but its occurrence in veins will not be 

 considered. 



(2) The minerals into which it may pass, giving their crystallizations, 

 specific gravities, and compositions. 



(3) The chemistry and physics of the processes of change, including 

 the volume relations. 



(4) The natural conditions under which the changes occur, and the 

 causes of the changes. 



With many minerals this outline can be carried out nearly to com- 

 pletion. With others the present state of knowledge is such that it can 

 be only very incompletely done. Consequently there is great variation 

 in the satisfactoriness of the discussion of the different minerals. When 

 the treatment of each of the minerals from these various points of view 

 can be carried out we shall have an interlocking system by which each 

 mineral is considered in its most important metamorphic connections. 

 To a certain extent the plan involves repetitions, but in each case the 

 important facts which concern an individual mineral are brought together. 

 The method of treatment proposed seems advisable, for many minerals 

 are both primary and secondary, and only by considering each mineral 

 from both points of view is it possible to understand the causes of the 

 changes as well as the changes themselves. Ordinarily the latter only are 

 considered. When one of the sources of a mineral is the alteration of 

 another the exact reactions concerned in the change are not given under 

 the former, but may be found by referring to the latter mineral which is 

 mentioned as its source. Ordinarily, however, qualitative statements are 

 made. To illustrate, a source of limonite is siderite. The reactions involved 

 in this change are to be found under siderite, not under limonite; but under 



