620 A TREATISE ON METAMORPHISM. 



calcite, garnet, and tourmaline have been observed, some of them frequently 

 In a clastic rock the process of enlargement means that the component 

 grains are minerals still and have the structure and potency of minerals- 

 These minerals, originally produced in igneous or metamorphic rocks, are 

 taken from their original positions and deposited in a secondary rock. The 

 beds are buried by overlying formations. Mineral-bearing solutions pass 

 through the new rock and each mineral fragment chooses from the solu- 

 tions material like itself. This it attaches to itself in optical continuity, 

 even though the time interval between the first and second growths be 

 indefinitely long-. In the early stages of this renewed growth crystal faces 

 are often rebuilt. (PI. IX, A.) If the growth continues, the enlarge- 

 ment of a particle meets similar enlargements from other grains. By 

 further growth these interlock and finally fill up 

 all the interspaces and perfectly indurate the rock. 

 fPl. IX, B.) 



This process of enlargement is of g'reatest 

 importance in the mechanical sedimentary rocks. 

 Its magnitude in these rocks is fully explained on 

 pages 865-868. The process is also important in 

 the volcanic fragmental rocks, including both coarse 

 -part of a thin section of a tuffs and ashes ; and, finally, the process does take 



quartz-schist showing liquid- and -i t ^-i -t . . . ,. -i -, . 



gas-mied cavities of a secondary place, although it is ot subordinate importance, m 



nature. Black Hills, South. Dakota. • • 1 rrii l •,! • i 



massive igneous rocks. I he cracks within and 

 between the grains may be cemented by substances the same as or different 

 from the materials cemented. If the cementation be imperfect secondary 

 gas- or liquid-filled cavities may be formed. Often the broken fragments 

 are somewhat displaced, but occasionally, in the case of parallel fractures, 

 the particles are broken apart and left very nearly in their original relative 

 positions, in which case the healing may be so perfect that the fact that the 

 rock has been fractured may be shown only by the gas- or liquid-filled 

 cavities and by the secondary inclusions. Thus are explained many of 

 the inclusions in cavities which are so frequently noticed to occur along 

 regular parallel planes. (See fig. 13.) 



In all cases where the process of cementation has gone far the depos 

 ited minerals interlock, and in the larger spaces they may have a coarsely 

 crystalline texture. The quantity of cementing material varies in different 



