﻿210 



PROP. S. J. SHAND ON 



[vol. lxxii, 



consisting of angular fragments of quartz and felspar, to very dense, 

 structureless, black bands the origin of which might have remained 

 obscure if they had not been associated with coarser granulitie 

 material. Dr. Clough informed me that these veins were at one 

 time considered to be of igneous origin ; in my view, however, 

 their true nature is unquestionably clearer than that of the Parijs 

 rocks. The specimens that exhibited the nearest approach to the 

 characters of the pseudotachylyte of Parijs came from Cunyan Crag 

 (Cheviots). Dr. Clough also showed me a flinty crush-rock from 

 South Ben Lee, North Uist (Hebrides), which, in regard to the 

 •extreme opacity of its base, almost defying microscopic resolution, 

 resembled the densest examples of my Parijs type 1. 



Dr. Clough, Mr. Maufe, & Mr. Bailey discuss flinty crush-rocks at 

 some length in their paper on the cauldron subsidence of den Coe, 

 and give a convenient summary of some of the literature mentioned 

 above. They show that at Glen Coe dark, apparently tachylytic, 

 rocks have been produced by the shearing of various t} r pes of rock, 

 quite independently of igneous action. Of especial interest is their 

 observation that in some cases ' the flinty crush-rock has been 

 injected as a fluid away from its source of origin.' By the 

 -courtesy of Dr. Flett and the late Dr. Clough and Mr. Bailey, I was 

 enabled to study a large number of hand-specimens and sections 

 from this well as from other parts of Scotland. I found 



that the majority of them were unmistakable mylonites, or eyed 

 gneisses on a microscopic scale ; but there seemed to be a re- 

 presentative of every stage between these and a type with almost 

 unresolvable dense base, which may show flow-structure or may lack 

 •eyery trace of orientation. Only the most extreme examples of the 

 latter type are comparable to the Parijs rocks, and none of them 

 showed anything similar to the crystallization of felspar and horn- 

 blende seen at Parijs. The points in which these rocks begin to 

 resemble those of Parijs are the following : — 



(1) The base of the rock is sometimes structureless and opaque, and 



full of tiny black grains, which are in part euhedral crystals of 

 magnetite. 



(2) Larger fragments are sometimes rounded, and have what look like 



reaction-rims, or their margins may appear to have been corroded. 

 Internally they may exhibit a mosaic structure. 



(3) In rare cases there are beginnings of crystallization in parts of the 



base, and in one instance (Meall Eiabhach, Ross-shire) typical 

 trichites are developed. 



Out of all the material that I examined I was able to select 

 only four specimens of which I could fairly say that they showed 

 a close approach to my Parijs rocks of the first type. 1 



Typical flinty crush-rocks are developed on a very large scale in 

 the province of Olavarria (Ai'gentine Republic), where they have 



1 The sections in the Geological Survey collection bear the numbers 12332, 

 13402, 13403, 12934, 4281. 



