OF CENTRAL CANADA PART I. 13 



The Irregular Forms assumed by minerals are of very subordinate 

 importance. The following are some of the more common : Globular 

 or nodular, ex. quartz, iron pyrites ; reniform or kidney-shaped, ex. 

 quartz, &c. ; botryoidal or mammillated : a form made up of a series 

 of rounded elevations and depressions, or otherwise exhibiting a sur- 

 face of this character, ex. red and brown iron ore, calcedony, &c. ; 

 stalactitic, ex. calc. spar, &c. ; coralliform, resembling certain branch- 

 ing corals, ex. arragonite ; acicular, in minute needle-like forms, ex. 

 millerite ; dendritic or arborescent, a branching form, often made up 

 of small aggregated crystals, ex. native silver, native copper, &c. ; 

 filiform or wire-like, ex. native silver. When a mineral presents a 

 perfectly indefinite shape, it is said to be massive. Other terms used 

 in connection with the irregular forms of minerals, such as incrusting, 

 disseminated, &c., explain themselves. The term amorphous is applied 

 to obsidian, opal, and other minerals in which crystalline structure 

 and cleavage planes are altogether wanting. 



Structure and Cleavage. In the majority of minerals a certain kind 

 of structure, or, in other words, the shape as well as the mode of 

 aggregation of the smaller masses of which they are composed, is 

 always observable. Structure in minerals may be either lamellar, 

 laminar or foliated, prismatic, fibrous, granular or compact. When 

 the mineral, as in most varieties of calc-spar, heavy-spar, feldspar, 

 and gypsum, for example, is made up of broad, tabular masses pro- 

 ducing a more or less stratified appearance, the structure is said to be 

 lamellar. When the tabular masses (whether straight, wavy or 

 curved) become extremely thin or leafy, as in mica more especially, 

 the structure is said to be laminar, or foliated, or sometimes mica- 

 ceous. The scaly structure is a variety of this, in which the laminae 

 are of small size. When the component masses are much longer 

 than broad or deep, as in many specimens of tourmaline, beryl, calc- 

 spar, &c., the structure is said to be prismatic or columnar. When 

 the prismatic concretions become very narrow, the fibrous structure 

 originates. Fibrous minerals may have either: a straight or parallel- 

 fibrous structure, as in many specimens of gypsum, calc-spar, &c. ; an 

 irregularly-fibrous structure, as in many specimens of augite and 

 hornblende; or a radiated -fibrous structure, as in the radiated varieties 

 of iron pyrites, natrolite, wavellite, and many other minerals, the 

 fibres radiating from one or more central points. Minerals made up 



