﻿part 3] 



THE PSEUDOTACHYLYTE OF PABIJS. 



211 



been studied by H. Backhand. In his memoir on the subject, 

 Backhand traces the mechanical alteration of a coarse porphyritic 

 granite into a black horny rock with relics of felspar-phenocrysts. 

 Optical deformation of the brecciated fragments is said to be visible 

 in every preparation, and as there is no obvious recrystallization it 

 is concluded that the breccia has not been submitted to any very 

 high temperature. Backhand compares these rocks with Scottish 

 flinty crush-rocks, which they appear to resemble closely. Four 

 •chemical analyses are tabulated, two of different facies of the 

 granite-gneiss and two of mylonites. The analyses of mylonite do 

 not differ more from the anahyses of granite than do the latter one 

 from the other, and. in particular, the analysis of a ' dense black 

 mylonite with red spots of microcline ' corresponds almost exactly 

 to the mean of the two analyses of granite-gneiss. 



The only other part of the world from which I have been able to 

 obtain useful information regarding crush-rocks and pseudotachy- 

 lytes is Little Namaqualand, the north-western corner of the Cape 

 Province. I am indebted to Dr. A. W. Rogers for the following 

 uote on the subject : — 



' The trap-shotten rocks in Van Rhyn's Dorp and Namaqualand are massive 

 gneiss and occasionally the mica-diorites of the copper-bearing intrusions. 

 The cracks are rarely half an inch wide, usually less than a tenth of an 

 inch. They — or, rather, the instances observed — are in groups several feet 

 long, and the individual cracks give off branches and join up with their 

 neighbours. Though they have no constant direction, the cracks in any one 

 group are parallel in a rough way, and at Jubilee Kop and on Mining- Lease 

 81 they cross the foliation of the gneiss approximately at right angles. 

 The strings are black.' 



In sections of one of these black veins from Flamink Vlakte 

 (Namaqualand) I was able to determine the following characters. 

 Under a low magnification the appearance seen is that of a powder 

 consisting of sharp angular grains of quartz and felspar of all sizes, 

 with a sort of cement of green scales. Even the highest mag- 

 nification fails to reveal anything of the nature of a glassy, 

 spherulitic, or microlitic base. The quartz and felspar fragments 

 are mostly free from cracks, and give a perfectly uniform extinction, 

 while the plagioclases show normal twinning lamellae. The green 

 scaly substance lying between the colourless grains is made up of 

 shapeless scales of biotite, with perhaps a few scraps of hornblende 

 of the same colour. This biotite seems to be mainly detrital, just 

 as the quartz and felspar are, but some of it may conceivably have 

 crystallized in place. There is absolutely no suggestion of corrosion, 

 melting, or recrystallization of the quartz or felspar, and nothing 

 whatever to indicate that the temperature was high. Magnetite, 

 which renders the Parijs rocks so dark and opaque, is nearly 

 absent. A few grains of zircon are visible, some of them being 

 enclosed in the larger biotite- scales. 



As an illustration of this mylonitic type of crush-rock, which 

 appears to be the standard type in all the regions above described, 



