﻿part 3] 



THE PSEUDOTACH YLYTE OF PARIJS. 



209 



V. COMPARISON OP THE PsEUDOTACHYLYTE WITH ' TpAP- 



Shotten Gneiss 1 and ' Flinty Crush-Rock.' 



The term trap-shotten (that is, trap-injected) gneiss was 

 employed by W. King & R. B. Foote for parts of the gneiss of 

 Salem (Madras), which they describe as being 'very largely im- 

 pregnated or shot with strings of dark green or bluish-black 

 compact trap. 1 The supposed trap-veins of this area were sub- 

 sequently studied by Sir Thomas Holland, who found them to 

 consist of an indurated black dust, through which fragments of 

 quartz and felspar are disseminated. He was able to imitate the 

 material by heating powdered gneiss to white heat in a furnace, 

 and he concluded that the production of the veins was due to the 

 brecciation of the gneiss along certain lines, but not to injection of 

 trap ; the black colour and indurated nature of the material he 

 ascribed to the action of heat produced during the violent brecciation 

 of the rock. 



The manner in which these dark veins occur in the field, as. 

 described and illustrated by the above-mentioned writers, recalls 

 many of the features of the Parijs occurrence. Important points 

 of difference, however, are that the so-called ' trap-shotten ' bands 

 occur in roughly parallel belts which coincide with lines of 

 dislocation, and that they are often associated with true igneous in- 

 trusions of a basic nature. With regard to microscopic characters, 

 the difference is fundamental ; in no single instance that I have 

 observed can the matrix of the Parijs rocks be described as a 

 ' black dust ' or powder. 



In Scotland rocks of somewhat similar characters to the above 

 have been described as ' flinty crush-rocks ' and mylonites.. Many 

 references to such rocks occur in the Geological Survey Memoir 

 on the North-West Highlands, and attention may be drawn espe- 

 cially to the descriptions on pp. 124, 221, and 249. It is again 

 significant that the veins are found only in highly dislocated 

 regions — especially along, or in the neighbourhood of, well-marked 

 fault-planes and zones of crushing. In most cases the flinty 

 material is associated with bands of a more clearly granulitic or 

 mylonitic character, and it has been suggested that b} r the intensity 

 of the crushing sufficient heat may have been generated to fuse 

 small portions of the rock. Some of the specimens in the Geolo- 

 gical Survey collection show the occurrence of black flinty material 

 as streaks within ordinary mylonite, apparently confirming the 

 above view ; a good example of this is Slide 12933. 



Dr. Harker has described crush-lines in granite near Broadford 

 (Skye), and notes that the most marked effects are restricted to 

 the vicinity of faulted boundaries. The flinty crush-rocks of the 

 Cheviot Hills, discovered by the late Dr. C. T. dough, also occur 

 along crush-lines in granite. Sections of these rocks, which 

 Dr. Clough kindly permitted me to examine^, show the clearest 

 evidence of shearing ; but they range from obvious mylonites, 



