CRYSTALLINE AGGREGATES. ol 
b. Concentric.When consisting of lamelle, lapping one 
over another around a centre, a result of successive concretion- 
ary aggregations, as in many concretionary forms, most pisolite, 
part of odlite, some stalactites, etc. 
Stratified, consisting of layers, as a result of deposition : 
é. g., Sime travertine, or tufa. 
d. Lb mded ; color-stratified. Like stratified in origin, but 
the layers usu ally 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. Lorms 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 fillang spaces that had been left occupied in con- 
sequence 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 
sunilar pseudomorphs after calcite, since shells and corals con- 
sist chiefly of calcite. Other quartz pseudomorphs have the 
form of fluorite, barite, ete. 
Again, the forms of calcite occur with the constitution of 
limonite, 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 spawn of 
serpentine, a hydrous magnesium silicate; and in this cause the 
ingredients of the serpentine silicate were present when the 
