'MATERIALS OF THE EARTH 43 



grains may go so far that the spaces between the grains are filled. 

 Sandstone thus changed becomes quartzite. Between lodse sand 

 at the one extreme, and quartzite at the other, there are all grada- 

 tions. " Quartzite is often regarded as a metamorphic rock, but it 

 is formed by a continuation of the process which cements sand 

 into sandstone. 



Grains of other minerals, also, such as feldspar, hornblende, 

 etc., are subject to similar secondary enlargement. Important 



Fig. 21. A, quartz crystal enlarged by secondary growth. The shaded 

 outline represents the outline of the sand grain; the solid lines, the out- 

 line after secondary growth. Magnified 33 diameters. B, sandstone 

 and quartzite texture. The shaded outlines represent the surfaces of 

 the sand grains before growth, the intervening white portions, the added 

 quartz, and the black portions, unfilled spaces. Open spaces charac- 

 terize sandstone. When the spaces are filled with quartz, the rock 

 becomes quartzite. Magnified 17 diameters. (Van Hise.) 



changes in rock are therefore brought about by solution, and 

 then the re-deposition of dissolved mineral matter, through the 

 influence of the water in the rocks. This process might be called 

 aqueous metamorphism, because of the important part played by 

 water. Since water is nearly always present in the rocks down 

 to considerable depths, the changes which water produces are 

 wide-spread, are, indeed, nearly universal, down to the depths 

 to which water penetrates, say five or six miles. 



Cavity filling. Cavities in rock larger than pores also receive 

 deposits, if the waters entering them carry mineral matter in 

 solution. The making of veins (p. 40) is a case in point. The 



