98 TERMINOLOGY. 



the roots of certain plants, the forms arising, are called tuberose, of 

 which Flint is one of the most common examples. 



To this class al>o, we must refer the irregular cellular shapes. 

 (. 80.) These distinctions are of little or no importance, for the 

 most part, as characters for the recognition of minerals; though 

 necer.sary to be understood, to prepare the pupil for the full descrip- 

 tion of the species, where it is intended that the descriptions shall 

 convey as complete a picture as possible of every individual inclu- 

 ded under each species. 



Those shapes which depend upon forms foreign to the mineral 

 kingdom, are the petrifactions. There is no difference between 

 the formation of the greater part of petrifactions, and of the pseudo- 

 morphoses, or the accidental imitative forms, and it does not there- 

 fore require any particular explanation. Mineralized organic re- 

 mains cannot be classed among real petrifactions. These are not 

 formed like pseudomorphoses, in which the space left empty by the 

 decomposition of one body is filled up by another, but the organic 

 mass is metamorphosed or changed into that of the mineral. Min- 

 eralized organic bodies, besides their original shape, may also retain 

 their original structure, as numerous vaiieties of Mineral Coal. 



Several minerals, even after their formation, assume other forms, 

 which are accidental. Such are pebbles, formed when fragments 

 of minerals are carried along by water, until, by attrition, they ac- 

 quire a more or less rounded shape. Simple, compound and mixed 

 minerals, are found in the shape of pebbles. 



.' 82. PARTICLES OF COMPOSITION. 



The individuals of which a compound mineral consists 

 are called its Particles of Composition. 



The particles of composition are true crystals, which, by their 

 contact, have prevented each other from assuming their regular form. 



The particles of composition have also been called Distinct Con- 

 cretions. The other expression, however, is preferable, since it 

 shows their reference to compound minerals, whereas distinct con- 

 cretions may also allude to simple minerals. 



The particles of composition are distinguished according to their - 

 length, breadth, and thickness, into granular, columnar, and lam- 

 ellar, particles of composition. The granular particles have all their 



