﻿126 
  

  

  CHAPTER 
  X. 
  

  

  Metamorphic 
  rocks. 
  

  

  The 
  rnetamorphic 
  rocks 
  lying 
  within 
  the 
  area 
  dealt 
  with 
  in 
  this 
  

   report 
  all 
  belong 
  to 
  one 
  great 
  series, 
  the 
  gneiss 
  series, 
  the 
  oldest 
  known 
  

   formation 
  in 
  India. 
  Although 
  they 
  occupy 
  a 
  considerable 
  area, 
  they 
  

   offer 
  but 
  little 
  of 
  interest 
  and 
  importance, 
  differing 
  in 
  this 
  respect 
  con- 
  

   siderably 
  from 
  other 
  gneissic 
  areas 
  to 
  the 
  west 
  and 
  south. 
  

  

  The 
  survey 
  of 
  the 
  gneiss 
  area 
  between 
  Madras 
  and 
  the 
  Palar 
  river 
  

   shows 
  that 
  the 
  various 
  beds 
  constitute 
  a 
  series 
  of 
  great 
  foldings 
  similar 
  

   to 
  those 
  worked 
  out 
  in 
  the 
  Salem, 
  Trichinopoly, 
  North 
  Arcot 
  and 
  South 
  

   Arcot 
  Districts,* 
  while 
  the 
  examination 
  of 
  the 
  country 
  south 
  of 
  

   the 
  Palar 
  by 
  the 
  late 
  Mr. 
  Charles 
  M. 
  Oldham 
  clearly 
  proves 
  that 
  

   character 
  to 
  be 
  continuous 
  across 
  the 
  space 
  intervening 
  between 
  the 
  

   country 
  included 
  within 
  sheet 
  79, 
  and 
  the 
  gneissic 
  area 
  north 
  of 
  the 
  

   Palar. 
  

  

  These 
  foldings 
  have 
  their 
  axes 
  generally 
  running 
  north- 
  north-east 
  

   to 
  south-south-west, 
  and 
  are, 
  on 
  the 
  whole, 
  very 
  regular, 
  but 
  owing 
  to 
  the 
  

   intervention 
  between 
  the 
  various 
  ridges 
  of 
  hills, 
  of 
  very 
  flat 
  valleys 
  

   thickly 
  covered 
  with 
  soil, 
  and, 
  in 
  many 
  cases, 
  with 
  thick 
  scrub 
  jungle 
  in 
  

   addition, 
  the 
  perfect 
  sequence 
  of 
  the 
  foldings 
  cannot 
  be 
  followed 
  out. 
  

  

  The 
  rocks 
  predominant 
  in 
  the 
  series 
  occurring 
  south-west 
  of 
  

   Madras 
  are 
  the 
  hornblendic 
  varieties 
  of 
  gneiss, 
  and 
  next 
  to 
  them 
  the 
  

   quartzose 
  varieties, 
  but 
  very 
  felspathic 
  rocks 
  are 
  of 
  great 
  rarity, 
  and, 
  

   except 
  in 
  one 
  instance, 
  of 
  small 
  local 
  extent. 
  Micaceous 
  gneiss 
  is 
  also 
  

   of 
  rare 
  occurrence. 
  

  

  The 
  foldings 
  are 
  most 
  perfectly 
  seen 
  along 
  the 
  rocky 
  hill 
  ridges 
  

   extending 
  from 
  Tirukarikunum 
  nearly 
  up 
  to 
  Vengambaucum. 
  They 
  are 
  

   also 
  well 
  developed 
  in 
  the 
  Chingleput 
  hills 
  and 
  in 
  the 
  group 
  of 
  rocky 
  

  

  * 
  Sue 
  Memoirs, 
  Geological 
  Survey 
  of 
  India, 
  Vol. 
  IV, 
  part 
  2, 
  passim. 
  

  

  ( 
  126 
  ) 
  

  

  