448 GEOLOGY, 



mica in preference to other silicates if the proper constituents are present, 

 and the result is that mica schists and gneisses, in which mica abounds, 

 are common products of the metamorphism of shales by contact with 

 bodies of lava. Mica schists and micaceous gneisses are also formed 

 in other ways, and other schists, dependent on the composition of the 

 shales, are formed about intrusions of igneous rock. In all such cases 

 pressure probably attends the heat and is a factor in the development 

 of the schists. When the change induced by the heat is less consider- 

 able, the shale is baked, with incipient recrystallization, and often takes 

 the form of argillite, a compact, massive sort of shale. 



Beds of hydrous iron oxide (Hmonite) or of iron carbonate (siderite) 

 are usually converted by heat into hematite or magnetite. Beds of 

 peat, lignite, and bituminous coal are converted into anthracite by the 

 driving off of the volatile hydrocarbons. If the process goes to the 

 extreme, graphite is the result. 



Metamorphism by heat and lateral pressure. — As already indicated, 

 the more common intense pressures experienced by rocks at and near 

 the surface are those that come from lateral thrusts arising from the 

 shrinkage of the earth. These affect one dimension of the rock-mass, 

 while they permit it to expand in one or both of the other dimensions. 

 This produces a strain in all the constituent particles of the rock, and 

 under such strain they pass more readily into solution than when free 

 from strain, and more readily rearrange their molecules internally into 

 positions of less strain. The crystals grow most freely along the planes 

 of least stress, i.e., at right angles to the pressure.^ As a consequence, 

 where unidimensional pressure and high heat resulting from the com- 

 pression unite their influence, the metamorphic changes are not only 

 facilitated, but the rearrangement is controlled by the pressure and 

 results in a parallel arrangement of the constituent crystals, giving a 

 foliated or schistose character to the new rock. The changes them- 

 selves are much the same as those produced by heat and water without 

 exceptional pressure, though some distinctions may be noted. It is to 

 be observed, however, that two kinds of work are embraced here: the 

 metamorphism of clastic rocks into crystalline schists, which may be 

 regarded as an upbuilding process, anamorphism, and the mashing 

 down of massive crystalline rocks into schists, which may be regarded 



^ The application of these principles we owe chiefly to Van Hise : Metamorphism 

 of Rocks and Rock Flowage, Bull. Geol. Soc. Am., Vol. 9, pp. 269-328. 



