200 HOLLAND : CHARNOCKITE SERIES. 



rock cooled from fusion. Besides, the structure is more often 

 found well beyond the range of any known trap-dyke. 



In some cases examination of very thin sections with a one- 

 tenth inch objective reveals the beginnings of an internal crystal 

 organization in the black dust ; but they are no more definite than may 

 often be seen in an indurated volcanic ash, and never of the nature 

 of a microlitic igneous matrix. The transparent fragments lying in 

 the black dust have generally the characters of quartz, all traces of 

 the felspars having in general been utterly destroyed. When mem- 

 bers of the charnockite series display this "trap-shotten " aspect, 

 microscopic examination of the fragments forming the breccia often 

 shows actinolitic fringes (similar to the sO'Called " reaction rims ") 

 around the hypersthenes, a phenomenon seldom exhibited in the 

 normal rock. Sometimes shallow bays in the quartz crystals are 

 filled with the black dust as if corrosion had commenced. Several 

 such phenomena indicate that the breccia has been highly heated, 

 but nevertheless not to a temperature sufficient to completely fuse 

 the dust. To check this idea experimentally, 1 crushed a specimen 

 of charnockite and heated the rough powder in a furnace to a white 

 heat, sufficient to produce a very imperfect fusion; the result was a 

 fritted black cake having the lustre of a tachylyte and showing in 

 thin sections a black structureless matrix including angular fragments 

 of quartz ; in fact, the fritted charnockite powder very closely resem- 

 bled the so-called strings of "trap" in these breccia bands. 



The mere heating of the charnockite dust — and in this ex- 

 periment the acid form was used — is thus evidently sufficient 

 to account for the black colour of the mylonite in the breccia without 

 any question of introducing material from without. 



The source of the heat which has indurated and blackened 

 the mylonite is then the only question left. The fact that there are 

 numerous instances of these " trap-shotten " bands well beyond the 

 range of intrusive dykes shows that the presence of the latter is not 

 essential ; and the perfectly unaltered condition of the rocks in the 

 neighbourhood of the brecciation bands shows that the effects of the 



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