384 PEOE. J. W. JUDD ON THE TEETIAET ANB 



bronzite and the typical hypersthene of Labrador " Schillerized fer- 

 riferous enstatites." 



The nature of the enclosures which give rise to the " Schiller " 

 phenomenon of minerals has been investigated by many mineralogists ; 

 but, as might have been anticipated when their minute size is taken 

 into account, the results arrived at are very discordant *. 



The enclosures vary in colour from grey to yellow, and through 

 various shades of brown to purplish red ; while they are sometimes 

 so dark as to be almost black and opaque. In all cases, so far as it 

 is possible to examine such minute objects, they prove to be isotropic. 

 As a general rule, they are found to be infusible, and to resist the 

 action of the strongest hydrochloric and other acids. 



In form, these enclosures greatly vary. Sometimes they appear 

 to have very definite outlines, which has led them to be regarded as 

 microscopic crystals of haematite, magnetite, brookite, augifce, or other 

 minerals ; but these identifications have not only not been sup- 

 ported, but in many cases have been actually disproved by chemical 

 analysis. jN'evertheless the regularity of their forms in the same 

 crystal, and sometimes a wonderful agreement in the angular 

 measures which they give, are very striking facts. In other cases, as 

 for example in the Labrador hypersthene, the tabular enclosures, 

 while presenting perfectly flat and parallel sides, exhibit the most 

 irregular contours along some of their edges, and their forms appear 

 to be quite irreconcilable with the hypothesis that they are micro- 

 scox^ic crystals. 



From a consideration of all that has been adduced by other in- 

 vestigators, taken in connexion with my own observations, I am led 

 to the conclusions that the substances forming these various enclosures 

 do not consist of any definite chemical compounds assuming the 

 regular crystalline forms belonging to mineral species, but that they 

 are mixtures of various oxides in a more or less hydrated condition, 

 such as hyalite, opal, gothite, and limonite ; hence their isotro- 

 pism, their variation in colour, and their resistance to the action 

 of acids. 



All Schillerized minerals on analysis yield a small but notable 

 proportion of water, which is probably contained in these enclosures. 



The suggestion which seems to me to be most in accord with all 

 the facts of the case, is that these enclosures are of the nature of 

 negative crystals which are more or less completely filled with products 

 of decomposition of the mineral. When these negative crystals are 

 completely filled with foreign substances, the enclosures assume the 

 outlines of true crystals, though they do not, of course, exhibit their 

 optical properties: it is noteworthy that in some cases a corre- 

 spondence between the angles of the enclosing mineral and the 

 inclusions seems to have been clearly made out. But in other 

 cases the secondary products are insufficient to fill the whole cavity 



* This subject has been especially treated of by Scheerer (Pogg. Annal. Ixiv. 

 1845) ; Vogelsang (Archiv Neerland. iii. 29, 1868) ; Eosmann (Neues Jahrb. f. 

 Min. &c. 1869, p. 532) ; Hagge (Mikroskop. Uutersuch. iiber Gabbro, &c. 1871) ; 

 Trippke (Neues Jahrb. f. JVIin. 1878, p. 676). 



