TERMINOLOGY. . 187. 



monly this appearance is explained by supposing them to 

 be real fissures, or that the coherence of the particles in 

 the rocky mass has, in their place, by whatever means, 

 been resolved. If a mineral is formed in a fissure of that 

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

 appearing in this shape, is said to occur in Plates. Mixed 

 minerals likewise may affect this shape ; and the veins 

 themselves might be quoted as examples, if their consi- 

 deration did not belong to another science. These fissures 

 sometimes are so very narrow, that a fluid can scarcely 

 enter between their sides ; a mineral formed in such a 

 space is said to occur superficial, which in fact is nothing 

 else, but a very thin plate. Examples occur in hexahe- 

 dral Silver, octahedral Copper, &c. both of plates, and of 

 superficial varieties. 



There are instances where the sides of these fissures 

 are nearly even, and possess a certain degree of polish. 

 Fissures of that* description very seldom seem to have 

 been filled up with other minerals : on the contrary, the 

 sides are commonly in immediate contact with each other. 

 Minerals are said to occur specular, if specimens of them 

 shew part of such polished sides of fissures. The specular 

 faces sometimes shew a particular sort of striae, which 

 would deserve very well to be noticed by geologists. As 

 examples, we may quote specular hexahedral Iron-py- 

 rites, hexahedral Lead-glance, brachytypous Parachrose- 

 baryte, &c. 



Several rocks contain vesicular cavities. In these ca- 

 vities minerals are formed, which consequently must as- 

 sume their shape, and appear as more or less spheroidal 

 masses. Sucii 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 

 (. 160.), and from the globules described above (. 183.). 

 Among the present we must class also the Agate-balls, and 

 the balls of other varieties of rhombohedral Quartz, as of 

 flint, of Egyptian jasper, &c. 



If this kind of globular concretions is not hollow inside, 



