g. 188. OF COMPOUND MINERALS. 265 



and at the same time very irregular, so as to exhibit some 

 resemblance with the roots of certain plants, the forms 

 arising are called tuberose ; of which flint is one of the 

 most common examples. 



To this class also we must refer the irregular cellular 

 shapes (. 188.). The present subject is of little value in 

 scientific Mineralogy, and does not require, therefore, very 

 nice divisions. 



Those shapes which depend upon forms foreign to the 

 mineral kingdom, are the ^!nf actions. There is no differ- 

 ence between the formation of the greater part of petri- 

 factions, and of the pseudomorphoses or the accidental 

 imitative forms, and it does not require any particular ex- 

 planation. Mineralized organic substances cannot be class- 

 ed 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. Mineralized organic bodies, besides their 

 original shape, also may retain their original structure, as 

 numerous varieties, particularly of bituminous Mineral-coal. 

 Several minerals, even after their formation, assume 

 other forms, which, however, are quite accidental. Thus 

 Pebbles are formed when fragments of minerals are carried 

 along by water, till, by attrition, they acquire a more or 

 less roundish or globular shape. Simple, compound, and 

 mixed minerals, are found in the shape of pebbles. 



. 188. PARTICLES O 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 (. 160.). 



The particles of composition have also been called .Dw- 

 tinct Concretions. The other expression, however, is by far 



