﻿part 3] 



THE PSEUDOTACHYLYTE OF PARI.TS. 



215 



rock was determined by geometrical measurement of four thin 

 sections ; the results were 25*8, 26"4, 28 - 2, and 43"5 per cent, 

 respectively, giving an average of about 30 per cent. Hence, in 

 order, to account for the observed composition of the pseudotachy- 

 lyte, we should be driven to the assumption that the supposed 

 basaltic ground-mass had actually dissolved and completely assimi- 

 lated about its own weight of granite. But a simple calculation 

 based on the known values of the specific heat and latent heat of 

 fusion of the common silicate-minerals shows that a magma, to 

 dissolve its own weight of foreign rock, would need to have an 

 initial temperature enormously in excess of any temperature known 

 to be realized in the higher zones of the lithosphere. 



If, then, the participation of an igneous magma is to be assumed 

 at all, it must be a magma considerably richer in silica and alkalies 

 than basalt. But here we are faced by the fact that the only body 

 of magma that is known to underlie the granite-gneiss in this 

 region is the body which gave rise to the enormous dolerite-dyke 

 at the weir above Parijs, and to the various ' diabases ' in the 

 Lower Witwatersrand rocks. The analysis, therefore, deprives 

 the igneous theory in its original form of an important buttress. 

 If we try to modify that theory by assuming that the invading 

 magma was of andesitic or dacitic composition, Ave merely beg the 

 question. 



VII. The Case against an Igneotts Oeigin. 



So long as no other than an igneous origin suggested itself, 

 certain peculiarities of structure and composition exhibited by 

 the pseudotachylyte could be overlooked ; but, Avith an alternative 

 solution in the field, these points of singularity must be regarded 

 as evidence against the igneous theoiy. The main points militating 

 against the acceptance of the igneous theory are as folloAVS : — 



(1) The total absence of macrocrystalline equivalents of the pseudo- 



tachylyte. 



(2) The total absence of differentiation between the central and the 



marginal portions of the veins. 



(3) The general absence of flow- structure. 



(4) The extreme opacity of the most glassy-looking rocks, and the 



abundance of powdery magnetite in them. 



(5) The absence of perfect isotropism, even in the most structureless 



types. 



(6) The low water-content of the rocks. Mr. E. A. Page determined for 



me the total loss on ignition in the case of one of the most nearly 

 glassy rocks (type 1) ; it amounted only to 0 - 83 per cent., which is 

 much lower than the average figure for pitchstones. 



(7) The melting (as distinct from dissolution) of felspar contacts. This 



is a phenomenon for which I can recollect no parallel among igneous 

 rocks ; it seems to demand a temperature higher than one can con- 

 cede to a thin tongue of magma far from its source. 



(8) The analogy between these rocks and the rare semi- vitrified crush- 



rocks of Scotland, and throiigh these with other Scottish occur- 

 rences of flinty crush-rocks and with the mylonites of Argentina, 

 India, and Namaqualand. 



n 2 



