COMPOUND MINERALS. 97 



The crystals of Steatite are considered as real crystals by some 

 mineralogists, and by others as pseudomorphoses; nothing decisive, 

 as respects this point, has as yet been brought forward. 



. 81. IRREGULAR ACCIDENTAL IMITATIVE SHAPES. 



According to the quality of the space, in which these 

 imitative shapes have been formed, they may be distin- 

 guished into: 1, those whose form is entirely accidental; 

 2, those whose form depends upon particular openings in 

 other minerals, which are not simple ones; and 3, those 

 whose form depends upon bodies, not belonging to the 

 mineral kingdom. 



In the mass of rocks, and in that of beds and veins, we very often 

 meet with cracks and fissures, which seem to have once been open, 

 or which still continue so. Commonly, this appearance is explained 

 by supposing them to be real fissures, or that the coherence of the 

 particles in the rftcky mass has, in their case, been overcome, by 

 some means or other. If a mineral is formed in a fissure of that 

 kind, it must necessarily assume its form; and the mineral, appear- 

 ing in this shape, is said to occur in plates. These fissures are 

 sometimes so very narrow, that a fluid can scarcely enter between 

 their sides; a mineral formed in such a space is safd to occur super- 

 ficial, which in fact is nothing else than a very thin plate. 



There are instances where the sides of these fissures are nearly 

 even, and possess a certain degree of polish. Fissures of this de- 

 scription very seldom seem to have been filled up with other miner- 

 als ; on the contrary, the sides are in immediate contact with each 

 other. The sides of such fissures are said to be specular. 



r Several rocks contain vesicular cavities. In these cavities min- 

 erals are formed, which consequently must assume their shape, and 

 appear as more or less spheroidal masses. Such globules very often 

 consist of the varieties of more than one species, and are sometimes 

 hollow inside. They must be accurately distinguished from the 

 grains, (. 54.) and from the globules described above. (. 77.) 

 Examples of this kind exist in Agate Balls, Quartz, &c. 



If this kind of globular concretion is not hollow inside, and at the 

 same time very irregular, so as to exhibit some resemblance to 



