222 
THE GEOLOGIST. 
Hematite (Fe) crystallize in nearly identical forms : the same is the 
case with Roch-saU (Na CI) and Fluor (Ca Fl) : and the Carbonates of 
Lime, Calcite(C& C), of Magnesia, Magnesile (Mg C), and of Protoxide 
of Iron, Chalahite (Fe C), have also similar crystalline relations. What 
is the case with these, and numerous other minerals occurring 
naturally, also holds good with many other bodies, which are only 
produced artificially in the laboratory. 
As isomorphous bodies are found to have the same form when they 
crystallize in an uncombined state, so it is likewise found, when they 
enter into combination as the components of substances of a more 
complicated chemical natiu-e, that the isomorphic property is still 
continued, and that they are capable of mutually replacing each other 
without causing any essential change in the crystalline relations of the 
compound substance. For example, Al and Fe are isomorphous, as * 
are likewise Ca and Fe. In the mineral Garnet, one variety is thus 
composed : Si 40 per cent., Al 25 per cent., and Fe 33 per cent. In 
another variety the Al and Fe are wholly replaced by their respective 
isomorphs Fe and Ca, the composition of the mineral being Si 40 per 
cent., Fe 29 per cent., and Ca 30 per cent. : and all this substitution 
without any change of crystalline form. 
Although isomorphic replacements do not alter the essential form 
of a mineral (which is its crystalline form), they often give rise to 
considerable changes in its other physical characters. This is pecu- 
liarly the case when the oxides of the heavy metals replace the earths 
and alkalies, giving rise to changes particularly in colour and specific 
gravity. The many varieties which characterise some mineral species 
are generally due to this. See the cases of Epidote, Garnet, Augite, 
Hornblende, &c. 
Among the large number of isomorphous bodies enumerated by 
mineralogists and chemists, those that concern us in rock-minerals are 
comparatively few. Those most worthy of attention are : — 
Elements Fl, CI. 
Compounds of 1 atom of base united j a. — Ca, Mg, Fe, Mn, K, Na, Li. 
with 1 atom of oxygen .... I h. — Ca (as Arragonite), Ba. 
Compounds of 1 atom of base \mited 
with 3 atoms of oxygen .... 
