82 PEOJT. J. W. JUDD ON TERTIARY 



But inasmucli as a crystal formed under one set of conditions may 

 be in a state of instability when those conditions are altered, we 

 frequently find the porphyritic crystals in the basic rocks exhibiting 

 evidence of this kind of action. 



It is in the basalt-glasses (tachylytes) and in the magma-basalts 

 which approach these in character that the most striking elBPects of 

 this kind are exhibited. As already pointed out *, the porphyritic 

 crystals of felspar, augite, and olivine in these rocks are completely 

 honeycombed by penetrations of glassy material identical in character 

 with the surrounding ground-mass. These porphyritic crystals are 

 certainly transported ones, and have been brought from below, where 

 they were formed under totally dijfferent physical conditions. En- 

 veloped in a mass of molten glass, differing in composition from that 

 in which they were formed, it is not surprising to find that they 

 have yielded to a great extent to the solvent action. 



It is interesting, as bearing upon the theory of schillerization, to 

 notice that this solution of crystals often takes place along definite 

 crystallographic planes ; that is to say there are certain planes of 

 chemical weakness in the crystals, a fact in complete harmony with 

 the phenomena of the " Aetzfiguren " and of "• schiUerization." 



§ 2. Action of Solvents under Pressure (Schillerization). 



This subject has been fully discussed in a former paper f. It is 

 there shown that as a result of the solvent action operating along 

 cracks or lines of strain, and also in certain planes of chemical 

 weakness, in crystals, secondary enclosures containing liquids or 

 solids are formed. Further, by the same agent the felspars acquire 

 avanturine and chatoyant properties, the pyroxenes are converted 

 into " Schiller " varieties, and the olivines are studded with stellar 

 inclusions, and finally become so filled with particles of magnetite 

 as to assume a black and opaque appearance. 



That these phenomena are due to deep-seated action is shown by 

 the fact that the schiller varieties of augite (known as diallage and 

 pseudo-hypersthene) and the " schiller" varieties of enstatite (known 

 as bronzite and hypersthene) are confined to the highly crystalline 

 and deeply-seated rocks ; and it is in such situations only that we find 

 the similarly altered forms of olivine and felspar. But as solvent 

 action at the surface may be local and partial in its action, so is it at 

 great depths ; and we find side by side in these deep-seated rocks 

 crystals which are more or less highly schillerized and others quite 

 free from this kind of change. 



That schillerization is the result of changes which have taken 

 place in crystals long after their formation, and that schillerized 

 minerals are therefore altered products, is proved in many ways. 

 The schillerizing process can be shown to have originated either 

 from the surfaces of crystals or along lines of accidental fissures 

 traversing the crystals, and the materials which are contained in 



* Quart. Journ. Geol. Soc. vol. xli. (1885) p. 444. 

 t Ibid. vol. xli. (1883) p. 383. 



