DESCRIPTION OF THE CHIEF TYPES. 163 



they are not precisely the same mineralogically in all the rocks— there 

 must be some agreement in the conditions under which they have 

 originated ; for when these needles appear in the garnets the 

 accompanying quartz crystals are always crowded with acicular 

 inclusions and the hypersthenes beautifully schillerized. When, 

 on the other hand, garnets are free of such acicular inclusions, the 

 quartz is also clear and the rhombic pyroxenes belong to the variety 

 which Professor Judd has referred to as proto-hypersthene. It seems 

 to me that the simultaneous appearance of these similar phenomena 

 in all the constituents of the rocks must be regarded as the result of 

 secondary causes, and the theory which is most in agreement with 

 all the facts of the case is that for which Judd has proposed the term 

 "schillerization." 1 



Although from the evidence obtained from hypersthene alone, 

 it may be difficult to show any great objection to the theory that the 

 schiller plates are the result of infiltration along cleavage planes ; 

 such a theory could not account for the regularity of crystallographic 

 disposition of the needles in the accompanying garnets and quartz, 

 nor could it account for the fact that the inclusions in the hornblende 

 of these rocks are arranged, not parallel to the cleavage planes, but 

 parallel to an axis of optical elasticity. It seems much more likely 

 that the action of secondary chemical agents displays itself by 

 phenomena developed in directions definitely related to the axes 

 which determine the crystal-form as well as the other physical 

 properties of the mineral. If, also, negative crystals are produced 

 by solvents acting more effectually along planes of chemical weakness, 

 then the composition of the mineral acted on will naturally influence 

 the nature of the product and of the substance forming the schiller 

 plate or rod. From this it follows that when the constituents of a 

 rock are schillerized, the rods, plates or needles will vary in compo- 

 sition according to the chemical nature of the mineral which is acted 

 on. 



* Quart. Journ. Ceol. Soc., Vol. XLI (1885), p. 408; Min. Mag., Vol. VII (iS86), 

 g>*3u 



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