﻿TRAP 
  DYKES. 
  31 
  

  

  Many 
  of 
  the 
  larger 
  dykes 
  are 
  strongly 
  magnetic, 
  owing 
  to 
  the 
  amount 
  

   of 
  magnetic 
  iron 
  entering 
  into 
  the 
  composition 
  of 
  the 
  rock. 
  

  

  The 
  ground, 
  in 
  the 
  south-western 
  part 
  of 
  the 
  Jurassic 
  field, 
  to 
  the 
  

  

  Dykes 
  in 
  Uinia 
  sand- 
  north 
  of 
  the 
  present 
  limit 
  of 
  the 
  trappean 
  area, 
  is 
  

  

  stones 
  - 
  closely 
  intersected 
  by 
  trap 
  dykes, 
  more 
  especially 
  

  

  in 
  the 
  neighbourhood 
  of 
  Than, 
  where 
  they 
  form 
  quite 
  a 
  tangled 
  network 
  

  

  on 
  the 
  surface 
  oJ^the 
  country. 
  

  

  Some 
  of 
  these 
  filled-in 
  rents 
  are 
  traceable 
  for 
  a 
  distance 
  of 
  30 
  miles 
  ; 
  

   others 
  may 
  be 
  equally 
  extensive, 
  though 
  not 
  continuously 
  exposed, 
  their 
  

   several 
  parts 
  not 
  being 
  visibly 
  connected. 
  

  

  While 
  certain 
  of 
  the 
  dykes 
  may 
  be 
  traced 
  to 
  within 
  the 
  trappean 
  

   area, 
  where 
  they 
  have 
  penetrated 
  vertically 
  through 
  300 
  or 
  400 
  feet 
  

   of 
  bedded 
  basalts, 
  others 
  do 
  not 
  persistently 
  reach 
  the 
  surface 
  even 
  of 
  

   the 
  sandstones. 
  The 
  annexed 
  sketch 
  of 
  a 
  section 
  in 
  the 
  bank 
  of 
  the 
  

   stream 
  between 
  Sarsana 
  and 
  Morthala, 
  north-west 
  of 
  Than, 
  will 
  illus- 
  

   trate 
  the 
  latter 
  instance. 
  The 
  dyke 
  here 
  shown 
  was 
  traceable 
  in 
  the 
  

   bed 
  of 
  the 
  stream, 
  and 
  again 
  further 
  on, 
  after 
  being 
  lost 
  on 
  the 
  surface 
  

   for 
  a 
  short 
  distance. 
  

  

  The 
  dykes 
  in 
  this 
  neighbour- 
  

   hood 
  are 
  mostly 
  of 
  large 
  size, 
  

   few 
  being 
  under 
  8 
  feet 
  in 
  

   width; 
  one 
  north 
  of 
  Than 
  

   measured 
  32 
  feet, 
  but 
  there 
  

   are 
  others 
  still 
  broader. 
  Occa- 
  

   sionally 
  a 
  partition 
  occurs 
  along 
  1 
  m 
  , 
  - 
  ra 
  * 
  

  

  J 
  * 
  ° 
  Sketcn 
  Section. 
  

  

  the 
  middle 
  of 
  the 
  course, 
  form- 
  

   ed 
  by 
  a 
  mass 
  of 
  sandstone 
  

  

  detached 
  from 
  the 
  walls. 
  The 
  walls, 
  as 
  a 
  rule, 
  are 
  vertical, 
  or 
  very 
  

   Slickensides 
  on 
  the 
  nearly 
  so, 
  with 
  clean 
  well-marked 
  faces, 
  and 
  often 
  

   sandstone 
  walls. 
  coated 
  with 
  silicious 
  slickensides, 
  as 
  are 
  also 
  the 
  

  

  sides 
  of 
  the 
  enclosed 
  masses. 
  1 
  

  

  1 
  Some 
  excellent 
  examples 
  were 
  obtained 
  of 
  enclosed 
  pieces 
  of 
  sandstone 
  with 
  slicken- 
  

   sides 
  on 
  either 
  side, 
  and 
  so 
  intensely 
  indurated 
  as 
  to 
  resembhvgneiss. 
  They 
  are 
  now 
  

   exhibited 
  among 
  the 
  physical 
  specimens 
  of 
  the 
  Survey 
  Museiua." 
  " 
  

  

  ( 
  103 
  ) 
  

  

  