286 



MATERIALS AND THEIR ARRANGEMENT 



brought about by the solution and rerdeposition of mineral matter 

 by the water in the rocks. This process may be called aqueous 

 metamorphism, because of the important part played by water. 

 Since water is present in almost all rocks down to considerable 

 depths, the changes which it produces are nearly universal down 

 to the depths to which it penetrates. 



Cavity filling. Cavities in rocks larger than the spaces between 

 grains also receive deposits, if the waters entering them carry min- 



Fig. 277. Veins of calcite in volcanic tuff. Shore west of Kincraig Point, Elie, 

 Fife. (H. M. Geol. Surv.) 



eral matter in solution. Thus joints or cracks may be filled with 

 mineral matter, making veins (Fig. 277). The agates developed in 

 some cavities afford another illustration of cavity filling. Here the 



successive layers are 

 commonly of quartz, 

 differing from one 

 another in color and 

 texture. Geodes are 

 cavities partly filled 

 with crystals (Fig. 

 278), mostly of 

 quartz or calcite. 



Replacements. In 

 both sedimentary 

 and igneous rocks 

 there are replace- 

 ments. Thus 

 through the dissolv- 

 ing and depositing 

 Fig. 278. Geode. (Bassler, U. S. Geol. Surv.) action of water the 



