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. Reticulated. — 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. 



b. Fibrous, when they are slender. 



c. Parallel fibres, when the fibres are parallel. 



d. Radiated, 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 columni 

 ending in projecting crystals. 



