METAMORPHISM OF SHALES. 903 



Simultaneously with the development of the aluminum-silicate series, 

 garnets and staurolites may be produced. The garnet usually marks a 

 stage of alteration less advanced than the staurolite, and thus it is that very 

 often schist-pelites and gneiss-pelites are garnetiferous and not staurolitic, 

 but in almost every case staurolitic schists and gneisses are garnetiferous. 

 The development of the garnets may use parts of any of the constituents, 

 calcium, magnesium, aluminum, iron, depending on the kind of garnet. 

 Common garnet, as explained in Chapter Y (p. 302), is usually an isomor- 

 phous mixture of grossularite, pyrope, and almandite, and occasionally 

 melanite. The particular garnet which develops in a given case largely 

 depends upon the elements which are present in the original rock. In case 

 iron is rather plentiful and the metamorphism is intense, staurolite is also 

 likely to form. The development of these minerals disposes of a large 

 amount of iron and alumina, iron being derived from the oxide, and alumina 

 in many cases being derived from the kaolin or from gibbsite or diaspore. 

 If exceptional elements are present, as, for instance, boracic acid, tourmaline 

 and other exceptional minerals may form. 



While in the majority of the schist-pelites and gneiss-pelites feldspar 

 is rather subordinate, in certain of them, as already noted, it is plentiful. 

 In proportion as the feldspar becomes abundant, the material from which 

 the rock was derived grades toward the quartz-feldspar-sand family of the 

 psammite order. 



During the metamorphism of the shales to slates, schists, and gneisses 

 the new mineral particles which develop have a very strong tendency to 

 dimensional arrangement. Indeed, the pelite order is the one which shows 

 this dimensional arrangement to the best advantage. The rocks here 

 belonging are the types of rocks of this class. The mineral of most 

 fundamental importance showing the dimensional arrangement is, of 

 course, mica, although some of the other minerals which develop show a 

 marked dimensional arrangement. That the eminent cleavage of mica 

 corresponds with its dimensional arrangement has been frequently men- 

 tioned heretofore, and is dealt with in detail by Leith." Consequent upon 

 the correspondence of the dimensional arrangement and cleavage of the 

 newly crystallized mica, the eminent cleavage of the slates, schists, and 



«Leith, C. K., Rock cleavage. 



