866 A TREATISE ON METAMORPHISM. 



stones, so in quartzites, iron oxides and calcite are subordinate cements, and 

 frequently other minerals to some extent perform a similar function. In 

 many parts of the continents the process of cementation has transformed 

 sandstone to quartzite for extensive areas, many such formations of widely 

 differing geological ages covering hundreds of thousands of square kilo- 

 meters. These quartzite formations vary in thickness in different places 

 from a few meters to as much as 3 or 4 kilometers. 



It has been seen under "Quartz-sand rock" (p. 863) that ordinarily the 

 material of well-sorted, pure sand formations does not occupy more than 

 two-thirds to three-fourths of the space. This requires that in the cementa- 

 tion of a sand to a quartzite from one-fourth to one-third of the formation be 

 added by the ground waters. It is therefore certain that the amount of 

 secondary silica required to indurate such great quartzite formations as 

 occur in the Paleozoic and pre-Paleozoic of the continent is enormous. 

 That required for vein fillings, while vast, is probably inconsiderable as 

 compared with this. 



For instance, if a quartzite formation were supposed to have a volume 

 of 100 cubic kilometers, to change this rock from a sand to a quartzite would 

 require the addition of from 25 to 33 or more cubic kilometers of quartz. 

 It would be very interesting to make an estimate as to the amount of quartz 

 which lias been added by the ground circulation to the quartzites which 

 are now known to exist, but the descriptions of quartzite formations are 

 not sufficiently accurate to admit of even an approximate estimate of their 

 volume. However, certainly many thousands, and probably hundreds of 

 thousands, of cubic kilometers of quartz must have been added by the 

 ground solutions in order to indurate these formations. It has been fully 

 explained on pages 480, 516-517 that the silica liberated from the 

 silicates by the process of carbonation in the belt of weathering is entirely 

 adequate to account for much or all of the silica supplied by the ground 

 waters. 



There remains to be considered the question where the process of 

 cementation of sand to sandstone and quartzite takes place. If the argu- 

 ment given on pages 856-857 in reference to the minor importance of the 

 cementation of psephites below the sea be correct, it is still more applicable 

 to the cementation of sands. Murray and Renard have estimated that sea 

 water contains in solution, on the average, only one part of silica in 250,000 



