622 A TREATISE ON METAMORPHISM. 



important are marcasite and pyrite, but from the point of view of ore 

 deposits many other sulphides are of great consequence. 



In addition to these, there are many subordinate minerals deposited in 

 the belt of cementation; indeed, almost every mineral which occurs in rocks 

 may there form, but the amounts are so small that they will not be 

 considered here. 



OXIDES. 



silica. — Silica may be deposited as opal, chalcedony, or quartz. In the 

 process of precipitation it may fill any of the classes of openings which exist 

 in rocks. But the greatest quantity of this material is deposited between 

 grains of sediments, and especially between the grains of quartzo'se sand. 

 Sandstone formations are one of the most abundant of the sedimentary 

 deposits. It has been shown on pages 124-126 that the original pore space 

 of such formations probably varied from one-fifth to two-fifths of the volume, 

 with a probable average of approximately one-third. The ordinary quartz- 

 ites which have not been modified by mechanical action are completely 

 indurated by the deposition of quartz between the grains, in orientation 

 with the original grains, or as independent material, or the two combined. 

 (PI. IX.) The volume of quartzite of this class now existing upon the 

 earth is very great. (See pp. 865-868.) In reaching a judgment as to the' 

 amount of quartz which has been precipitated in sand, it must be remem- 

 bered also that many sandstone and quartzite formations produced in 

 past geological times have been destroyed by the forces of erosion and 

 redeposited. 



One of the reasons why quartz is so very extensively deposited in 

 quartzose sandstone is furnished by the principle (see pp. 120-122) that, 

 other things being equal, material in solution is precipitated where there 

 are nuclei of the same composition upon which it may be deposited. 

 The innumerable rounded grains of quartz in the sands furnish the neces- 

 sary nuclei which promote the precipitation of silica. 



But quartz is deposited not only between the grains of quartzose sands; 

 to some extent it is deposited between the grains of all other porous 

 mechanical sediments, although the amount thus precipitated is subordinate. 

 A vast amount of quartz is also deposited in the vacuoles of volcanic rocks 

 as amygdules. Further, silica is deposited in the innumerable openings of 

 the fissile rocks, in the numberless joints, and along the numerous faults 



