THE PRESENT STATUS OF MINERALOGY. 801 



elusions drawn from analytical results. Proof of homogeneity is an 

 essential datura in the establishment of a mineral species. Many a 

 supposed species has been overthrown by the microscope. 



But a mineral may be apparently homogeneous, and yet indefinite 

 chemically ; for, apart from mere impurities which are unrecognizable 

 by physical means, there are modes of admixture even more difficult 

 to determine. Two distinct compounds may crystallize together in 

 varying proportions, so as to yield definite forms which are, to all 

 physical tests, perfectly homogeneous. Such mixtures of "isomor- 

 phous " substances are almost infinite in number ; they are among the 

 commonest occurrences in Nature ; and they complicate the miner- 

 alogic problem enormously. Theoretically, a species is easily defined ; 

 practically, the definition is most troublesome. Oftentimes all the 

 members of an isomorphous group are regarded as one species, in which 

 certain analogous elements are said to " rej^lace " each other. Iron 

 and alumina are thus mutually replaceable ; so are the oxides of the 

 magnesia group ; so also are sodium and potassium. But this usage, 

 though common and sanctioned by weighty authorities, is not rigidly 

 scientific. It is allowable conventionally, but only so long as we do 

 not lose sight of what it really means. The so-called replacement is 

 in reality a phenomenon of mixture between isomori^hous salts of allied 

 metals or acids, which salts are the true, definite species. For example, 

 garnet varies in composition in just this peculiar way, and six or more 

 compounds, all different but similar, are represented in it. Some- 

 times we find one of these compounds nearly pure ; but oftener two 

 or more exist in a given crystal. Garnet, therefore, is not one species, 

 but a group, and should be so treated. A mixture is a mixture, wheth- 

 er visibly so or not, and has no title to specific naming. On a sys- 

 tematic basis the current policy needs modification ; for it varies too 

 widely and can not be universally applied. Seeking to evade one 

 set of difficulties, it creates new ones. 



In consequence of the tendency toward mixture among species, 

 and of the wide-spread fashion of regarding the crystal as the mineral- 

 ogical unit, there has grown up a general belief that minerals are in 

 most cases very complex chemically. Some species, undoubtedly, are 

 quite simple, like quartz, fluor-spar, or calcite ; but others, especially 

 among the silicates, appear to be most complicated, and even variable, 

 in composition. This complexity, which is in great part due to the 

 influences already mentioned, is perhaps apparent rather than real. 

 Mixtures, whether crystalline or mechanical, can hardly be given either 

 simple or definite chemical formula. The true individual units are 

 probably not very complex, for their modes of origin favor simplicity. 

 A complex molecule is likely to be unstable — the more complex, the 

 more unstable ; while minerals seem to be generated under conditions 

 adverse to instability. Some have been deposited from solutions in 

 which many reactions were possible ; others originate under conditions 



TOL. XXXII. 51 



