GENERAL CONSIDERATIONS. I99 



the Salem gneisses. On the supposition that the black compact 

 strings and shreds were due to injected basic trap they suggested the 

 name "trap*shotten gneiss 1 ." 



Microscopic examination of the rocks, however, does not oiict 

 evidence in support of this theory, although, judging from mac- 

 roscopic characters alone, the conclusion is a most natural one. 

 In the first place, the black substance possesses none of the peculiar 

 microscopic structures which characterise {< trap " or any substance 

 which has resulted by direct consolidation from thorough fusion ; it 

 is, on the other hand, composed of a black dust, through which angular 

 fragments of quartz and other transparent minerals are disseminated, 

 and the whole rock is highly crushed, with the production of mylon- 

 ite and frequent microscopic faulting of the constituents. Very 

 often this brecciation is quite evident in the field and is accom- 

 panied by a well-marked " strain-slip " cleavage in the neighbouring 

 rocks. In fact, the phenomenon is essentially a form of brecciation 

 due to dislocation of the rocks along definite lines. 



But the production of the strings and tongues of compact, 

 black mylonite is a peculiarity for which I can recall no exact paral- 

 lel amongst crush phenomena ; for the rocks are often acid in com- 

 position and that at first makes the black colour of the mylonite a 

 matter of considerable surprise. Careful exmination by the micros- 

 cope shows that where the quartz crystals have been smashed and 

 granulated, the granular bands often include innumerable minute 

 opaque, black bodies which suggest either sublimation by heat or 

 introduction of material in solution. Herein comes the significance 

 of King and Foote's observation that the so-called trap-shotten phe- 

 nomena are accompanied occasionally by true trap-dykes. Where 

 trap-dykes are actually associated with this structure it is possible 

 that compounds may have been sublimated into the adjoining 

 breccia; but it is quite certain that the black substance is not a 

 bodily injection of molten material : the black material has essentially 

 the microscopic structure of an indurated dust, never that of a 



1 Op. cit.y p. 271. 

 G (■ 81 ) 



