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Transactions. 



project from the weathered surface, and can readily be removed from 

 the soft matter that encloses them. These crystals exhibit to perfection 

 the form of augite, but when sections of some of them were made they 

 all proved to consist of a core of colourless augite surrounded by a margin 



of uralite. 



These facts are of great significance. If the strike of these dykes be 

 continued it is found that they may be expected to appear as outer 

 members of the hornblende-schist series lower down the harbour. There 

 is, therefore, no doubt that the amphibolite is a continuation of these 

 dykes, but that, being in closer proximity to the norite, it has suffered 

 metamorphism and uralitization to a greater degree. 



No lengthy explanation of the name applied to this rock need be 

 offered, as all authorities use the term for rocks " more or less markedly 

 schistose in which hornblende is the dominant mineral." 



The chemical composition is given below : — 



SiO, 49-75 



Ti0 2 



A.UO3 .. 



Fe 2 0, 



FeO 



MnO .. 



MgO .. 



CaO .. .. •• • - " 



K.,0 .. 



Na,0 



Loss on ignition . . 



Total 



101-75 



Specific gravity, 2-94. 



The Contact of the Amphibolite and tin. Hornblende Schists. — At the 

 line of junction these two rocks are more easily recognized in hand- 

 specimens than they are under the microscope, for the causes that pro- 

 duced the metamorphism in both tended to bring about an exchange of 

 material between the two. Thus the amphibolite is richer in feldspar 

 where it is in contact with the schists, and the schists are relatively 

 enriched with hornblende. 



THE PROCESS OF URALITIZATION. 



So far as I can ascertain, no writer has yet put forward an 

 exact definition of uralite, and there does not seem to be a con- 

 sensus of opinion as to what varieties of secondary hornblende are 

 covered by the term. For instance, Harker,* in discussing the 

 decomposition of augite, says, " Another common alteration is the 

 conversion to hornblende, which may be light green and fibrous (uralite) 

 or deep brown and compact." On the other hand, Williamsf mentions 

 the fact that the uralite fringing the pyroxenes " exhibits a marked 

 tendency to become compact along its outer edge." Again, GeikieJ terms 

 uralitization " the conversion of pvroxene into compact or fibrous horn- 

 blende." 



* Harker, A., " Petrology for Students." 4th ed. (1908), p. 70. 



t Williams, G. H., Am. Journ. Sci., vol. 28 (1884), p. 261. 



% Geikie, "Text-book of Geology." vol. 2, 4th ed. (IOCS), p. 790. 



