﻿part 3] 



THE PSEUDOTACHYLYTE OF PARIJS. 



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I found the differences to be at least as numerous as the resem- 

 blances, it became clear that I must reconsider the features of the 

 Parijs occurrence in the light of the evidence derived from other 

 regions. Parijs was accordingly revisited during 1915, and such 

 fresh information and illustrations as were gained have been em- 

 bodied in the account which is now presented. 



It should be explained that the name pseud otachylyte has 

 been adopted in recognition of the fact that these rocks have a 

 great similarity to tachylyte, also that such rocks have been mis- 

 taken for trap and tachylyte in Scotland and India as well as in 

 South Africa, and for the further reason that no more suitable 

 name is in existence. ' Trap-shotten gneiss ' denotes the entire 

 complex of intrusion and intruded rock, and is misleading in view 

 of the fact that the intrusive part is not certainly ' trap ' at all ; 

 while ' flinty crush-rock ' begs the question — as regards the Parijs 

 rocks, at least. 



II. Occurrence m the Field. 



The township of Parijs lies upon the northern portion of the 

 Vredefort granite-mass and on the southern bank of the Vaal 

 Paver. The granite, which has an outcrop of some 400 square 

 miles, has generally been regarded as Archaean (that is, pre-Wit- 

 watersrand), and appears as such on Dr. F. H. Hatch's map of the 

 Transvaal ; but Dr. Molengraaff, and more recently Mr. F. W. 

 Penny, have held that this granite is in reality intrusive in the 

 Witwatersrand System. No detailed survey of the region has yet 

 been made. 



The ' granite ' in the neighbourhood of Parijs is a streaky 

 granitic gneiss, composed of red and grey elements. Sometimes 

 the red forms patches and streaks within the grey, elsewhere the 

 grey matter is similarly enveloped by the red, or again the two 

 elements may constitute alternate bands. The red matter often 

 forms veins and bands of coarse pegmatite which run parallel to the 

 direction of foliation of the grey rock, but in other cases such veins 

 cut sharply across the foliation. These pegmatites are occasionally 

 very coarse-grained graphic granites. When extensive exposures 

 are studied, it becomes evident that the red portion is of later con- 

 solidation than the rest of the rock. Isolated ' floaters ' of banded 

 grey paragneiss can be found embedded in the red granite; and, to 

 my mind (although I have not made a special study of the gneiss- 

 granite), the matter is susceptible of one interpretation only, as 

 follows : — the grey facies of the granitic gneiss results from impreg- 

 nation, metamorphism, and eventual assimilation of sedimentary 

 country-rock by the ascending magma, while the red is the residual 

 portion of the same magma. Probably neither part reproduces the 

 initial composition of the magma exactly. 



The red bands are composed essentially of quartz and felspar, 

 with a very small proportion of biotite. Cleavage-pieces of the 

 red felspar show it to be a true orthoclase ; the red coloration is 



q2 



