140 HOLLAND : GEOLOGY OF THE NEIGHBOURHOOD OF SALEM. 



along the south-east face of the Shevaroys. Near Salem and to the 

 north-eastward the rocks show a well marked " strain-slip " 

 cleavage x and the peculiar phenomenon which King and Foote 

 named l( trap-shotten " gneiss on the supposition that the strings 

 and tongues of hardened black mylonite were injected "trap, " 

 which they really do closely resemble in microscopic characters. 

 In the neighbourhood of Salem the most pronounced of these so- 

 called " trap-shotten " bands runs N.-E.— S.-W., and in some places 

 accompanies a very thorough brecciation of the rocks. Good 

 examples may be seen showing the direction of the brecciation 

 bands in" a field at about | mile south of the seventh milestone from 

 Salem on the old Cuddalore road ; on the western spur of Jarugamalai, 

 •3 miles south of Salem, and further to the south-west on the west side 

 of the Namakal road. The first-mentioned locality is not far from 

 the western end of the strip of leaf-gneisses described in a previous 

 section of this paper (pages 8 to 14), and in fact the abrupt ending of 

 these gneisses along a line running N.-E. from this point suggests 

 that they have been cut off by a subsidiary N.-E. — S.-W. fault. 



These so-called "trap-shotten" bands have been more fully 

 discussed in a memoir on the charnockite series, 3 where it is shown 

 that the black colour and compact character of the mylonite may be 

 artificially imitated by crushing up charnockite and raising it to a 

 white heat in a furnace. At a white heat the dust begins to frit and 

 at the surface becomes glazed, forming a hard black cake which 

 in microscopic characters closely resembles the black tongues of 



Bonney, Quart. Journ., Geol. Soc, XLII (1886), 95. Synonyms '. — Ausweichungs- 

 clivage (Heim, " Mechanismus der Gebirgsbildung," II, 53) J a spurious or pseudo-cleavage 

 as opposed to the true schistose structure or ultimate-structure cleavage of Sorby {cf. Harker, 

 Brit. Assoc. Report for 1885, p. 836). 



2 Op. cit., p. 271. 



3 Mem., Geol. Surv., Tnd., Vol. XXVIII, pt 2. Although this peculiar phenomenon was 

 studied first in connection with charnockite, and has been described in the writer's memoir on 

 that series in South India, the same structure is found in other gneisses and has no known 

 petrological limitations. 



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