94 TERMINOLOGY. 



&c. Under these circumstances, they assume the shape of beds, 

 veins, &c., the consideration of which forms the province of Geology. 



. 79. ACCIDENTAL IMITATIVE SHAPES. 



The accidental imitative shapes presuppose an empty 

 space, which has been filled up by the individuals of com- 

 pound minerals, to which is transferred the form of the 

 the pre-existing space. 



In this case, the shape, which the mineral assumes, is not a con- 

 sequence of the properties inherent in the mineral, but is due solely 

 to the space in which its formation takes place, the sides of which 

 serve for a support to the individuals. Thus, at first a coating is 

 formed which consists of small, but, in many cases, very percep- 

 tible crystals, -whose apices are turned towards the inside of the 

 empty space. This accounts for the hollowness of many imitative 

 forms of this kind, of which the cavities are still lined with crystals. 



The space, in which the accidental imitative shapes are produced, 

 may be either regular or irregular. A regular space cannot be 

 produced except by crystallization ; and this may be either in the 

 interior of a real crystal, or it is the cast of a crystal in the surround- 

 ing mass. The first is not uncommon, particularly in large crys- 

 tals of Quartz, where part of the space of the crystals has remain- 

 ed empty, and is regularly limited by the surrounding crytalline 

 mass. 



The irregular spaces sometimes consist of accidental fissures, 

 cracks, and other similar openings; sometimes they depend upon 

 the structure of the surrounding mass; others are derived from the 

 moulds of various minerals, and also of organic bodies. 



The different kinds of space above alluded to, produce a distinc- 

 tion of their forms into regular and irregular accidental imitative 

 forms. 



. 80. REGULAR ACCIDENTAL IMITATIVE SHAPES. 

 PSEUDOMORPHOSES. 



The regular imitative shapes have been called pseudcn 

 morphoses, or supposititious crystals. 



