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PROCEEDINGS OF THE GEOLOGICAL SOCIETF. 



many additions to these curious anomalies in the optical behaviour 

 of minerals have been made by other observers. So greatly, indeed, 

 have these been multiplied in recent years, that it is doubtful if any 

 mineral crystallizing in the Cubic, the Tetragonal, or the Hexagonal 

 system could be cited in which the optical properties are precisely 

 what they ought to be according to theory; and similar anomalies 

 are also found in crystals possessing lower degrees of symmetry. 



The attempts which have been made by some crystallographers to 

 account for these optical anomalies in crystals, by assuming that 

 they possess only a pseudosymmetry, the result of very complicated 

 twinning, ingenious as they undoubtedly are, remind one of the 

 wonderful addition of eccentrics and epicycles by which astroDomers 

 so long sought to maintain the credit of the Ptolemaic theory. But 

 as in the latter case complexities and difficulties alike vanished when 

 the centre of the system was shifted from the earth to the sun, so 

 have the discoveries of Klein and Rosenbusch removed the necessity 

 for the painfully elaborate crystallographic hypotheses to which I 

 have referred. 



Most mineralogists will now be prepared to admit, as the result 

 of these researches, that the perfection alike of form and of optical 

 properties which may characterize a crystal when first formed, is 

 liable to slight modification, as the conditions of temperature and 

 pressure under which it exists vary. In consequence of this, almost 

 all natural crystals are found, when we study them with sufficient 

 care, to exhibit slight but very striking and significant differences in 

 form and optical behaviour from what they ought theoretically to 

 possess. 



While our knowledge of the ordinary mineral varieties promises 

 to be vastly extended by the improvements which have been made in 

 the methods of optical and chemical diagnosis under the microscope, 

 there is nevertheless, at the same time, reason to hope that the 

 relationship of these numerous varieties will, by the same means, be 

 made more distinctly apparent. As the existence of well-defined 

 natural groups of minerals becomes more clearly established, through 

 the study of interesting though inconspicuous links, we shall obtain 

 a basis for a much-needed reform in Mineral Taxonomy and Nomen- 

 clature. 



The more carefully we pursue our researches among the diversified 

 forms of the mineral world, the more are we impressed by the con- 

 viction that each mineral, like each plant or animal, possesses 

 its own individuality. Nature does not make facsimiles in the 



