CRYSTALLINE AGGREGATES. 61 



6- Concentric. — When consisting of lamellae, lapping one 

 ovet another around a centre, a result of successive concretion- 

 ary aggregations, as in many concretionary forms, most pisolite, 

 part of oolite, some stalactites, etc. 



c. Stratified, consisting of layers, as a result of deposition : 

 f e. g sc me travertine, or tufa. 



d. Handed ; color-stratified. Like stratified in origin, but 

 the layers usually indicated only by variations in color ; the band- 

 ing is shown in a transverse section : e. g. } agate, much stalag- 

 mite, riband jasper. 



e. Geodes. — When a cavity has been lined by the deposition 

 of mineral matter, but not wholly filled, the enclosing mineral 

 is called a geode. The mineral is often banded, owing to the 

 successive depositions of the material, and frequently has its 

 inner surface set with crystals. Agates are often slices or frag- 

 ments of geodes. 



6. Forms derived from the crystals of other minerals. Pseu- 

 domorphs. — Crystalline aggregates, especially the granular, 

 sometimes have forms derived from the crystals of other 

 minerals either 



(1) Because a result of cotemporaneous removal and substi- 

 tution : or 



(2) Because a result of the alteration of such crystals; or 



(3) Because filling spaces that had been left unoccupied in 

 consequence of previous removal. 



For example. Crystals occur having the forms of calcite 

 (calcium carbonate, or " carbonate of lime "), but consisting of 

 quartz or silica. They were made from calcite crystals by the 

 action of some solution containing silica, the solution dissolving 

 away the calcite and depositing at the same time silica or quartz. 

 Specimens occur showing all stages in the change from the ear- 

 liest in which the calcite is thinly coated with quartz, to the 

 last, in which it is all quartz. Such crystals are pseudomorphs 

 of quartz after calcite. Siliceous fossil shells and corals are 

 similar pseudomorphs after calcite, since shells and corals con- 

 sist chiefly of calcite. Other quartz pseudomorphs have the 

 form of fluorite, barite, etc. 



^gaiu, the forms of calcite occur with the constitution of 

 limonile, a hydrous iron oxide. In such a case the iron oxide 

 was in the solution that corroded and dissolved away the 

 calcite. 



Again, the forms of calcite occur with the constitution of 

 serpentine, a hydrous magnesium silicate ; and in this case the 

 ingredients of the serpentine silicate were present when the 



