630 MESSRS. CXOTJGH, HAUTE, AXD BAILEY OX [NoV. [909, 



In fact, these strings are probably nothing else but extreme types 

 of flinty crush-rock, although some of them (4281, 5270), 1 as might 

 be expected from their field-relations, reveal under the microscope the 

 beginnings of a crystalline structure in their ground-mass and so 

 indicate that they were at one time partly fused. The enclosures 

 in this ground-mass are fragments of the neighbouring rock, and they 

 show the same advanced cataclastic structures as those that are 

 typical of the enclosures in the flinty crush-rocks occurring along 

 definite shear-zones in the vicinity. 



Flinty crush-rocks are known as ' trap - shotten ' among the 

 charnockite gneisses in Southern India, and their nature has been 

 elucidated by Sir Thomas Holland 2 : — 



' The so-called " trap-shotten " bands coincide with lines of dislocation, and 

 the black tongues and films which superficially resemble compact " trap " have 

 the microscopical characters of mylonite which has been hardened — fritted 

 and rarely half-fused — by the heat generated through the dislocation being 

 confined to narrow bands, and thereby causing a higher local rise of temperature 

 than would result from a general deformation of the rock-mass.' 



By crushing a specimen of charnockite, and then subjecting it to 

 a very imperfect fusion, Sir Thomas Holland was able to reproduce 

 experimentally the blackness, tachylytic lustre, and structureless 

 base, containing numerous angular enclosures of quartz, which are 

 so characteristic of the ' trap-shotten ' bands in the charnockite 

 gneisses. 



The evidence in Glen Coe corroborates the conclusions which 

 have been arrived at in the districts mentioned above. It can be 

 shown that here, as elsewhere, dark flinty or tachylitic rocks with 

 perfect flow-structure have been produced by the shearing of various 

 types of rock, quite independently of igneous action of any sort 

 (p. 650). A clear example is also known, in which flinty crush- 

 rock of normal structure has left the fault-plane with its accom- 

 panying shear-phenomena, and has been injected as a viscous body 

 into the surrounding schists (p. 653). 



When a microscopical investigation is made into the nature of 

 the sheared material which accompanies the flinty crush-rock, it 

 is found that this material has not always one particular struc- 

 ture. In the North-West Highlands the materials of the dis- 

 integrated rock sometimes show marked deformation and strain- 

 structures, which become obvious when a thin slice is examined 

 between crossed nicols. Here the intermediate product is apparently 

 somewhat mylonitic in character ; and this also appears to be the 

 case with the ' trap-shotten ' bands of India, so far as can be judged 

 from the descriptions. In the Cheviot and Glen Coe examples, how- 

 ever, mylonization is not prominent; but a trituration, resulting in 

 the production of a pseudo-grit, is apparently the first stage in 



1 The numerals in parentheses refer to the corresponding numbers of the 

 microscope-slides in the Geological Survey collection. 



2 ' The Charnockite Series, a Group of Arcbrean Hypersthenic Eocks in 

 Peninsular India' Mem. Geol. Surv. India, vol. xxviii (1900) pp. 198, 248. 



