44 GEOLOGY 



agates developed in some cavities afford another illustration. 

 Here the successive layers are commonly silica (chalcedony, p. 76), 

 and differ from each other in color and texture. Before the cavity 

 is entirely filled, the deposit may change from chalcedony to crystals 

 of quartz which grow with their pyramidal points toward the 

 center of the cavity. Geodes are examples of a similar process 

 in which the cavity is but partially filled with crystals (Fig. 22). 

 The crystals of geodes are most commonly quartz or calcite, but 

 they may be any other mineral that the waters are capable of 

 depositing. Large cavities lined in this way are known to miners 

 as vugs, and these grade into caves with linings of crystals, and 

 with stalactite and stalagmite (Fig. 23). 



When small cavities- in rock are filled by material deposited 

 from solution, the result is sometimes called a secretion. Crystal- 

 lined cavities (geodes, Fig. 22) and agates are examples of secre- 

 tions. Crystal-lined cavities and veins are the same in principle. 



Replacements. In both sedimentary and igneous rocks there 

 are replacements, sometimes resulting in imitative or false forms. 

 Thus the calcium carbonate of corals, shells, etc., may be replaced 

 by silica. This substitution may take place in such a way that 

 the minutest details of structure are preserved. Similarly, woody 

 matter is sometimes replaced by silica, forming silicified or petri- 

 fied wood. 



Pseudomorphs. Again, the molecules of one crystal are some- 

 times replaced by different material, as the molecules of calcite 

 by zinc carbonate, giving a pseudomorph of zinc carbonate after 

 calcite. The zinc carbonate takes the form of the calcite, instead 

 of the form which it would take if crystallizing under other circum- 

 stances. Hence the name. This sort of change may affect the 

 crystals in any sort of rock. 



Concretions. A notable phase of the internal reconstruction 

 of sedimentary rocks is the assembling of matter of the same kind. 

 For instance, silica that was probably deposited in the form of 

 siliceous shells and spicules of plants and animals, and dissem- 

 inated through the sediments as they were deposited, is later 

 aggregated into nodules of chert or flint (Fig. 24). Similarly, 

 concretions of calcium carbonate or iron carbonate grow in silts 



