VIEWS ON MINERAL SPECIES. 441 



tend to assume certain molecular proportions as in Dolomite. 

 It is generally assumed now, in consequence of the 

 researches of Dufet and Retgers, that the former is the 

 case, and that truly isomorphous compounds can graduate 

 into each other by imperceptible variation of mixture. 



Be this as it may, we are obviously compelled to admit 

 that minerals such as Calcite and Dolomite, though not 

 isomorphous in the strictest sense, possess a similarity both 

 of form and of composition, and, whether their similarity 

 be indicated by the term species, genus, order, group, or 

 family, they must be brought together in any system of 

 rational classification of which the object is to emphasise 

 and to reveal the resemblances of minerals. 



The term species is freely used in mineralogy, but in 

 very different senses. Many authors, in particular Berzelius, 

 Rammelsberg. Fuchs, and Groth, would banish the word 

 entirely from this science. Obviously if Garnet is a species 

 so is the whole group, including Calcite, Magnesite, etc. ; 

 on the other hand, if Calcite is a species so is each of the 

 Garnets mentioned above, together with the other 

 compounds conforming to their type of composition 

 R" 3 R'" a (Si 4 ) 3 ; so that if R" may be Ca, Mg, Fe, or Mn, 

 and R"' may be Al, Fe, or Cr, there are at least twelve 

 species belonging to the Garnets. 



Dana, in the fourth and preceding editions of his 

 system, defined a mineral species as " any natural inorganic 

 substance composed of particles capable in favourable 

 circumstances of combining by means of their mutual 

 attractions so as to constitute a crystalline solid". He goes 

 on to say : " The question still arises, What is distinctively 

 a mineral species? Since it has been found that in accord- 

 ance with the principles of isomorphism there are elements 

 which may replace one another indefinitely and still the 

 form of the crystals remain the same, crystallisation has 

 been assumed as the only authoritative test of identity or 

 distinction of species ; and this idea has been the means of 

 greatly simplifying the science ; the species Garnet would 

 make, according to the old chemical view a dozen or more 

 species. . . . Research may yet discover a principle 



