THE GEOLOaiST 



JUNE, 1859. 



ON ROCKS ; THEIR CHEMICAL AND MINERAL COMPOSI- 

 TION, AND PHYSICAL CHARACTERISTICS. 



By H, C. Salmon, Esq. Plymouth. 



{Continued from jtage 69.) 



XXI. Relation between the Physical Characters op a 

 Mineral and its Chemical Composition. — In the definition which 

 I have given, a mineral is described as a body possessed of a definite 

 chemical composition and a regular physical form, — meaniug by the 

 latter particularly crystalline structure. But, while the crystalline 

 relations of each species only vary within the narrowest limits, the 

 chemical composition has a much wider latitude. Many individuals 

 of the same mineral species are found, by analysis, to differ most 

 widely in their chemical components, while the crystalline form is 

 quite unaltered. Now, although we are as yet unable to trace the 

 relation between the form of a mineral and its composition, there can 

 still be little doubt that some such relation does exist, regulated by 

 laws yet to be discovered ; consequently, when we find the same 

 mineral species differing widely in chemical proportions, we recognise 

 a departure from regular although unknown laws, and seek for a 

 cause. This we find in the doctrine of Isomorjjhism. This, simply 

 stated, is the capability of two or more substances, of analogous 

 chemical constitution, to crystallize in similar forms. Thus, as a few 

 examples in the mineral kingdom, we find that Corundum (Al) and 



VOL. II. s 



