42 GEOLOGY 



METAMORPHISM AND METAMORPHIC ROCKS 



We have already seen that igneous rocks undergo physical and 

 chemical changes, whereby they are disintegrated, giving rise to 

 what has been called rock waste; but the waste from one generation 

 of rock is the raw material for rock of a new generation. It is " rock 

 waste " in somewhat the same sense that lumber is forest waste. 



Properly speaking, all changes which rocks undergo after being 

 formed are metamorphic changes. According to this view, de- 

 cayed rock is a phase of metamorphic rock; but it has been cus- 

 tomary to limit the term "metamorphic" to those rocks which, 

 instead of being disintegrated by the changes which they undergo, 

 are rendered more compact, more complex in constitution, or more 

 crystalline. Both sedimentary and igneous rocks may be meta- 

 morphosed. 



Induration of sediments. The first step in the alteration of 

 sediments is their induration, through the aid of cement, pressure, 

 etc. Sandstone and shale are not commonly called metamorphic 

 rocks, but they may be called metamorphosed sand, and metamor- 

 phosed mud, respectively. The cementing material of sediment 

 is mineral matter deposited from solution in water. Thus material 

 dissolved at and near the surface may be carried down by de- 

 scending water, and deposited between the grains of sediment, 

 binding them together. In the process of cementation, a striking 

 plan is often followed, as illustrated by the cementation of sand 

 by silica. It is to be remembered that the common grains of sand 

 are more or less rounded particles of quartz, derived from the 

 imperfect crystals of some rock which contained quartz. The crys- 

 tallization of quartz, or any other mineral, involves the arrange- 

 ment of the molecules in some definite way, peculiar to each 

 mineral. When new silica is deposited from solution about these 

 rounded grains of quartz sand, it is always deposited in such a way 

 about each grain as to help to build it out into the form which 

 crystals of quartz always take if free to grow as they will. Micro- 

 scopic examination of well cemented sandstone often shows the 

 original grains, distinct from the cementing material which appears 

 as additions to original grains (Fig. 21). The enlargement of the 



