CRYSTALLINE AGGREGATES. 59 
dendrites. They have been formed by crystallization from a 
solution of mineral matter which has entered by some crack and 
spread between the layers of 
the rock. They are often 
black, and consist of oxide 
of manganese; others, of a 
brownish color, are made of 
limonite; others, of a red- 
dish black or black color, of 
hematite. Moss-like forms 
also occur, as in moss agate. 
c. feticulated. — Slender 
prismatic crystals promiscu- 
ously crossing, with open 
spacings. 
d. Divergent.—Free crys- 
tals radiating from a central 
point. 
e. Drusy.—A surface is drusy when made up of the extremi- 
ties of small crystals. 
2. Consisting of columnar individuals. 
a. Columnar, when the columnar individuals are stout. 
6. Librous, when they are slender. 
c. Parallel fibres, when the fibres are parallel. 
d. Ladiated, when the columns or fibres radiate from 
centres. 
e. Stellated, when the radiations from a centre are equal 
around, so as to make star-like or circularly-radiated groups. 
f- Globular, when the radiated individuals make globular or 
hemispherical forms, as in wavellite. 
g- Botryoidal, when the globular forms are in groups, a lit- 
tle like a bunch of grapes. The word is from the Greek for a 
bunch of grapes. 
h. Mammillary, having a surface made up of low and broad 
prominences. The term is from the Latin mammilla, a little 
teat. 
i. Coralloidal, when in open-spaced groupings of slender 
stems, looking like a delicate coral. A result of successive ad- 
ditions at the extremity of a prominence, lengthening it into 
cylinders, the stems generally having a faintly radiated struc- 
ture. : 
Specimens of all these varieties of columnar structure, except- 
ing the last, often have a drusy surface, the fibres or columns 
ending in projecting crystals. | 

