386 PROF. J. W. JUDD 01!^ THE TERTIAE,r AK"D 



certain optical constants, such as the position of the plane in which 

 the optic axis lies and the angle between the optic axes *. 



The researches by which the optical constants of minerals have 

 been determined have proved that not only do the colour and cha- 

 racteristic pleochroism disappear in the substance of crystals t 

 which have undergone Schillerization, but that the position of the 

 optic axes and the angle which they make with one another are also 

 affected. This remarkable effect of the Schillerizing process is shown 

 if we compare unaltered and altered examples of minerals of the 

 same chemical composition with one another, augites with diallages 

 and pseudo-hypersthenes, ferriferous enstatites with bronzites and 

 hypersthenes. Tschermak even points out that the optical constants 

 of diallage are practically the same as those of diopside J. In the 

 latter mineral ferrous oxide was almost absent from the first ; in the 

 former it has been to a great extent removed from the substance of 

 the crystal and collected into the enclosures during the process of 

 Schillerization. 



. § 6. The Agency by which the ScHiiLEEizATiOiSr of Mesterals has 



BEE]^ effected. 



In seeking for the causes which have produced in minerals 

 the very remarkable changes which we have grouped together under 

 the name of Schillerization, there are two very important facts 

 which must be borne in mind. In the first place such changes are 

 quite distinct from those which result from weathering action, or 

 the penetration of water from the surface. Under the influence of 

 this kind of action, felspars are more or less completely kaolinized, 

 and their elements may subsequently recrystallize as zoisite and 

 other minerals ; augites are converted into uralite or directly into 

 hornblende, and olivines and enstatites into serpentine, steatite, 

 and talc. But in the minerals of the rocks we are describing, it is 

 manifest that crystals which do not exhibit the smallest trace of 

 Schillerization may be completely altered by weathering action ; and, 

 conversely, crystals which are perfectly fresh and uudecomposed may 

 have undergone the most striking effects of Schillerization. In cases 

 where the results of weathering action have been superinduced upon 

 those following from Schillerization, very complicated phenomena 

 may be presented, which it may require much care to unravel. 

 Cases of this kind we shall proceed to consider in the second part of 

 this paper. But the examples of the Western Isles of Scotland are 



* See for the pyroxenes, Tscliermak, Mineral. Mitth. vol. i. (1870) ; 

 Wilk. Zeitschr. f. Kryst. toI. viii. p. 208 (1884) ; Dolter, Neues Jahrb. f. Min. 

 &c. 1885, vol. i. p. 43. 



t It is true that slices of hypersthene viewed with a dichroiscope appear strongly 

 pleochroic. But when examined with a high power, the substance between the 

 enclosures is seen to be nearly colourless, and to exhibit only faint traces of 

 pleochroism. In examining the whole slice of the mineral the light transmitted 

 by the brown enclosures is affected by the intermediate substance, and a general 

 effect of pleochroism is produced, which is not seen in either crystals or enclosures 



I Lehrbuch fiir Mineralogie (1883), p. 440. 



