. 21. PRELIMINARY OBSERVATIONS. 21 



ral of these properties, they cease to be suitable ob- 

 jects for the consideration of Natural History. 



A mineral possessed of the properties it received from the 

 power of crystallisation, is in its natural or original state 

 (. 3.). A mineral which has lost these properties more or 

 less, is decomposed, and ceases to be an object of natural- 

 historical consideration. 



Minerals thoroughly decomposed commonly appear in 

 the form of powder, or as shapeless masses, without present- 

 ing any regular structure, or lustre, or determined and con- 

 stant degrees of hardness or specific gravity ; and the co- 

 hesion of their particles is destroyed. They form part of 

 the friable or earthy minerals. An example of an earthy 

 mineral we have in Porcelain-earth, a substance produced by 

 the decomposition of prismatic Feld-spar.* The decompo- 

 sition of minerals, however, does not in all cases proceed so 

 far. Some minerals retain their form, whilst colour, lustre, 

 hardness, &c. are changed ; as in several varieties of hexa- 

 hedral and prismatic Iron-pyrites. All, even the slightest, 

 of these alterations, exercise an influence upon the natural- 

 historical consideration of those bodies. It is in direct oppo- 

 sition to the principles of Natural History, to consider de- 

 composed varieties of one species, as varieties of another ; 

 but this, nevertheless, has been but too often the case in 

 Mineralogy. Thus, decomposed varieties of hexahedral and 

 prismatic Iron-pyrites, and of brachytypous Parachrose- 

 baryte, have been taken for varieties of prismatic Iron-ore. 

 In most cases it is possible to determine what the decom- 

 posed minerals have been in their natural or original state, 

 though indeed, for that purpose, we have often to recur to 

 considerations foreign to Natural History. 



It seems that the substance of several minerals has, in 

 the period of their formation, not arrived at that state of 

 perfection which distinguishes the finished productions of 



* Another class of friable minerals consists of very small 

 fragments of crystals, and grains of fresh or not decomposed 

 minerals. Such are fine sand, &c. 



