408 PROF. J. W. JT7DD ON THE TEETIART AND 



by which this change has been brought about, the name of " Sehilleriza- 

 tion " is proposed, and diallage and common hypersthene are shown 

 to be Schillerized forms of augite and ferriferous enstatite respec- 

 tively. In the case of some minerals, and notably the rhombic 

 pyroxenes, the altered forms are much more familiar to minera- 

 logists than the unaltered. 



The enclosures which exist in these Schillerized minerals, giving 

 rise to their peculiar colour, lustre, and sheen, are of the nature 

 of negative crystals, more or less completely filled with products of 

 decomposition, such as hydrated silica and hydrated ferric oxide. 

 When these isotropic mixtures fill the whole cavity of the negative 

 crystal, the enclosures appear to have definite crystalline forms ; 

 in many instances, however, they form patches with more or less 

 irregular outlines, partially filling the hollow of the negative crystal ; 

 and sometimes the crystalline forces have come into play and have 

 caused them to assume " dendritic " forms within the cavities where 

 they are deposited. These negative crystals, with their contents, 

 vary greatly in size, from objects visible to the naked eye down to 

 such as can only just be recognized by the highest powers of the 

 microscope, while there are probably others which are ultramicro- 

 scopic in their dimensions. 



The production of the Schillerized condition in minerals is shown 

 to be related to the depth at which the crystals have originally 

 existed in the great central cores of the Hebridean volcanoes. The 

 Schillerized forms of the minerals are only found in deep-seated 

 intrusive rock-masses ; but the converse of this statement is not true, 

 for in deep-seated rocks this change is sometimes evidently local, and 

 some of the crystals may have altogether escaped it. The degree 

 of Schillerization increases also with the depth at which the rock 

 has existed. 



An efficient agent for the production of this Schillerization, that 

 is the formation of negative crystals and their more or less complete 

 infilling with decomposition-products, is pointed out in the solvent 

 action of heated water and other fluids acting under great pressure. 



This solvent action takes place most readily along certain planes 

 within the crystal, and these directions of greatest susceptibility to 

 chemical action differ from those of easiest fracture (cleavage-planes) ; 

 the positions of such planes are perhaps also dependent to some extent 

 on twin-structure, facts for which we were prepared by the closely 

 related phenomena of the Aetzfiguren. We have here, in fact, the 

 phenomena of the Aetzfiguren seen in three dimensions — that is to 

 say, displayed in a solid instead of on a surface. 



In some cases the secondary products contained in the negative 

 crystals seem to be derived from the mineral in which the hollows 

 occur ; in other cases, as in the felspars, it is clear that they must 

 have been, in part, brought from outside the crystals afi'ected. 



The partial solution of minerals which results in the formation of 

 negative crystals within them often brings about great changes in 

 the colour, in the pleochroism, and in the positions and relations of 

 the optic axes of the original crystals. It is probable that the iron- 



