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PUOF. S. J. 8 HAND ON 



[vol. lxxii,. 



except Parijs, I include a photomicrograph of the rock of Flamink 

 Ylakte in PI. XIX, fig. 6. The essential difference between pseudo- 

 tachylyte and flinty crush-rock is impressively shown when this 

 photograph is compared with PL XVIII, fig. 2. 



The facts that have been established so far may conveniently be 

 tabulated in the following form : — 



(a) Black rocks are composed of crushed material without recrystal- 



lization or any evidence of elevated temperature : Argentine ; 

 Namaqualand ; most Scottish and Indian localities. 



(b) Black rocks are composed of crushed material, but with evidence 



of high temperature approaching the melting-points of some of the 

 constituents, and with beginnings of crystallization : some Scottish 

 localities, especially Glen Coe, Meall Biabhach, and North Uist ; 

 some Indian localities (?). 



(c) Black rocks hold inclusions of fragmental material, but lack proof 



of origin by crushing ; the temperature exceeded the melting-point 

 of felspar, and spherulitic and microlitic crystallization took place •. 

 Parijs. 



Regarding the evidence from a purely qualitative standpoint, it 

 would seem that we have a complete series of rocks connecting up 

 the pseudotachylyte of Parijs with ordinary mylonite, the various 

 links in the series being as follows : — 



mylonite -> fritted mylonite or flinty crush-rock ->• fused mylonite or 

 pseudotachylyte (type 1) -> recrystallized pseudotachylyte (types 2 

 & 3). 



Arguing on these lines, it is possible to maintain the view that 

 pseudotachylyte is simply an extreme form of flinty crush-rock, 

 the production of which involved a greater generation of heat than 

 usual. 



Against this view of the origin of pseudotachylyte there are 

 some very weighty arguments. The supposed series of rocks 

 connecting pseudotachylyte with mylonite, although qualitatively 

 complete, is very imperfect when examined quantitatively. In all 

 the regions that have been mentioned here, except Parijs, the first 

 member of the supposed series is abundant, the second member much 

 scarcer, the third member is limited to a few minute or even 

 microscopic occurrences, while the fourth member does not occur. 

 At Parijs, on the other hand, the first and second members are 

 entirely absent (at least a close search has failed to reveal them), 

 while the third and fourth members occur in great abundance 

 throughout a large area. These considerations cast the gravest 

 doubt upon the actual continuity of the supposed series. In fact, 

 the only conclusion that the study of flinty crush-rocks entitles us 

 to draw is that products similar to pseudotachylyte can be, and 

 in rare cases have been, produced by crushing and heating 

 associated with movement along planes of dislocation. 



Further objections to the comparison of pseudotachylyte with 

 flinty crush-rock are as follows : — 



