﻿220 



PROF. S. J. S1IAND ON 



[vol. lxxii, 



Fig. 4. Spherulitic ground-mass, type 3. X 80 diameters. Crossed nicols. 

 (See p. 206.) 



5. Ground-mass of felspar-microlites, type 3. X 100 diameters. Crossed 



nicols. (See p. 206.) 



6. Flinty crush-rock from Flamink Vlakte (Namaqualand). X 15 dia- 



meters. (See p. 211.) 



Discussion. 



Sir Jethro Teall considered that the paper was an important 

 contribution to a most interesting and difficult subject. The facts 

 had been very clearly described by the Author. He (the speaker) 

 had examined many specimens of 'flinty crush' from Scotland, and 

 had been much puzzled to account for the phenomena. The bits 

 of quartz and felspar which often formed a large part of the rock 

 showed marked signs of crush and strain, while the dark matrix 

 rarely exhibited any signs of crystallization. The flinty material 

 occurred in thin veins which sometimes anastomosed. He found 

 it very difficult to understand how the material was produced, and 

 how it came to be distributed in this way through the rock from 

 which it was largely if not entirely produced. 



Mr. J. F. N. Green said that, whatever might be the case with 

 the occurrences which had been quoted from India and other places 

 outside South Africa, there did not appear to be any evidence for 

 crushing in the description that had just been given. Except for 

 the character of the country-rock, the macroscopic phenomena were 

 of a kind familiar to students of the volcanic areas of this country. 

 He placed on the table two specimens from the Lake District 

 showing, on a small scale, brecciation,' comminution, blind veins, 

 etc., closely resembling the photographs and drawings exhibited. 



Mr. E. Greenly drew attention to some specimens and slides 

 of ' flinty crush-rock ' from the Lewisian Gneiss of Scotland that 

 were exhibited on the table, and had been kindly lent by the 

 Geological Survey. When he first met with these veins in the 

 Loch Maree country, five-and-twenty years ago, he certainly 

 supposed that they could be nothing but igneous injections. Soon 

 afterwards, however, on the other side of the lake, he found that 

 the same kind of material, appearing first as veins in almost 

 uninjured gneiss, increased rapidly in quantity towards a powerful 

 zone of^crush near the lake-side, until, in that zone, the gneiss was 

 permeated by it in all directions. The dynamic origin of the 

 material (already advocated by Dr. Clough) was here unquestion- 

 able. One of the slides on the table showed microlithic structure 

 in a vein -rock, another showed crushed gneiss of the lake-side zone 

 breaking down and passing into ' flinty ' matter. 



Dr. A. Holmes said that, having now heard the revised version 

 of the Author's paper, he was still inclined to support the original 

 suggestion that the pseudotachylyte veins were of igneous origin. 

 The form of the intrusion could be paralleled in Mozambique, 

 where the speaker had seen irregular veins of granulitic granite 

 penetrating gneiss in an equally intricate fashion. If the pseudo- 

 tachylyte were the result of crushing in situ, then every gradation 



