94 LYELL'S ELEMENTS OF GEOLOGY. 



Theory of Isomorphism. 



lute identity. For there is often some portion of the materials 

 in a crystal which are not in perfect chemical combination with 

 the rest. Carbonate of lime, for example, sometimes carries 

 with it a considerable quantity of silex into its own form of crys- 

 tal, the silex being mechanically mixed as sand, and yet not pre- 

 venting the carbonate of lime from assuming the form proper to 

 it. This is an extreme case, but in many others some one or 

 more of the ingredients in a crystal may be excluded from per- 

 fect chemical union ; and after fusion, when the mass recrys- 

 tallizes, the same elements may combine perfectly or in new 

 proportions, and thus a new mineral may be produced. Or some 

 one of the gaseous elements of the atmosphere, the oxygen, for 

 example, may, when the melted matter reconsolidates, combine 

 with some one of the component elements. 



The different quantity of the impurities or refuse above 

 alluded to, which may occur in all but the most transparent and 

 perfect crystals, may partly explain the discordant results at 

 which experienced chemists have arrived in their analysis of the 

 same mineral. For the reader will find that a mineral deter- 

 mined to be the same by its physical characters, crystalline 

 form, and optical properties, has often been declared by skilful 

 analysers to be composed of distinct elements. (See the Table 

 at p. 102.) This disagreement seemed at first subversive of the 

 doctrine, that there is a fixed and constant relation between the 

 crystalline form and structure of a mineral, and its chemical 

 composition. The apparent anomaly, however, which threat- 

 ened to throw the whole science of mineralogy into confusion, 

 was in a great degree reconciled to fixed principles by the dis- 

 coveries of Professor Mitscherlich at Berlin, who ascertained 

 that the composition of the minerals which had appeared so va- 

 riable, was governed by a general law, to which he gave the 

 name of isomorphism (from to^oj, isos, and f^op^rj, morphe, form). 

 According to this law, the ingredients of a given species of 

 mineral are not absolutely fixed as to their kind and quality ; 

 but one ingredient may be replaced by an equivalent portion of 

 some analogous ingredient. Thus, in augite, the lime may be in 

 part replaced by portions of protoxide jDf iron, or of manganese, 

 while the" form of the crystal, and the angle of its cleavage 

 planes, remain the same. These vicarious substitutions, how- 

 ever, of particular elements cannot exceed certain defined limits. 



Having been led into this digression on the recent progress of 

 mineralogy, I may here observe that the geological student must 

 endeavour as soon as possible to famiharize himself with the 

 characters of five at least of the most abundant simple minerals 

 of which rocks are composed. These are, felspar, quartz, mica,' 



