438 SCIENCE PROGRESS. 



Under this scheme, which closely resembles that of 

 Rose, the so-called isomorphous groups are successfully 

 retained undivided. 



A great merit of Dana's fourth and succeeding editions 

 was that minerals which have been proved to be the same, 

 or only to differ in unimportant particulars, such as a slight 

 variation of some constituents, are brought together under 

 one name. No new name is invented, but in accordance 

 with the law of priority, one of the names previously used 

 is applied to the mineral as a species, while the others are 

 retained either as the names of varieties or as synonyms ; 

 they can never be eradicated from the literature, and 

 hence it is important to keep them in their proper 

 place. 



This excellent plan, though it apparently deprives 

 Mineralogy of system, or at any rate of systematic nomen- 

 clature, and preserves only trivial names which are often 

 uncouth or absurd, is really the only plan which can be 

 satisfactorily adopted at the present time. It possesses the 

 necessary elasticity which enables us to transfer a given 

 mineral from one species to another if it be proved to be 

 more closely related to the latter. 



The principles of classification are comparatively easy so 

 soon as the method of Dana and Rose is accepted ; every 

 mineral specimen is supposed to possess a definite chemical 

 composition and a definite crystalline form ; those which 

 resemble one another both in composition and form are 

 brought together into a "group". Thus the rhombohedral 

 carbonates Ca CO s , Mg CO s , Zn CO s , Mn C0 3 , Fe C0 3 are 

 five minerals similar in composition and in crystalline form, 

 and they are brought together in the system into one group ; 

 so also are the four garnets Ca 3 Al 2 (Si 4 ) 3 , Ca 3 Cr 2 (Si 4 ) 3 , 

 Ca 3 Fe 2 (Si 4 ) 3 , Fe 3 Al 2 (Si 4 ) 3 . 



In reality the scheme is not so easy of application for 

 two reasons : ( i ) because minerals are never so perfectly 

 simple in composition but graduate into one another, so 

 that it is difficult to say where a given species begins and 

 where it ends; (2) because the groups are so few in number, 

 and there are hundreds of minerals which have to remain 



