﻿Vol. 
  53.] 
  ON 
  ROCKS 
  FROM 
  THE 
  BALUCHISTAN 
  BORDER. 
  295 
  

  

  but 
  also 
  a 
  very 
  wide 
  diurnal 
  range. 
  Our 
  solar 
  radiation 
  -thermo- 
  

   meter 
  would 
  register 
  205° 
  F. 
  on 
  cloudless 
  days, 
  while 
  the 
  nights 
  

   in 
  the 
  sandy 
  tracts 
  were 
  often 
  bitterly 
  cold. 
  The 
  diurnal 
  variation 
  

   must 
  sometimes 
  have 
  been 
  as 
  much 
  as 
  150° 
  F. 
  It 
  is 
  needless 
  to 
  

   point 
  out 
  how 
  powerful 
  an 
  agent 
  these 
  wide 
  variations 
  of 
  daily 
  and 
  

   annual 
  temperature 
  must 
  be 
  in 
  the 
  disintegration 
  of 
  the 
  surface 
  of 
  

   the 
  country. 
  Nor 
  is 
  there 
  cause 
  for 
  astonishment, 
  if 
  we 
  think 
  of 
  

   the 
  peculiar 
  conditions 
  of 
  water, 
  wind, 
  and 
  sand-action 
  and 
  of 
  heat 
  

   and 
  cold 
  obtaining 
  in 
  this 
  country, 
  that 
  its 
  surface 
  should 
  present 
  

   curious 
  and 
  unusual 
  features. 
  

  

  Part 
  II. 
  — 
  Petrological 
  Notes 
  on 
  the 
  Rocks. 
  

   By 
  Lieut.-Gen. 
  C. 
  A. 
  McMahon. 
  

  

  A 
  number 
  of 
  rock-specimens 
  collected 
  by 
  my 
  son, 
  Capt. 
  A. 
  H. 
  

   McMahon, 
  CLE., 
  Boundary 
  Commissioner, 
  when 
  engaged 
  in 
  the 
  

   delineation 
  of 
  the 
  boundary 
  between 
  Baluchistan 
  and 
  Afghanistan 
  

   up 
  to 
  the 
  borders 
  of 
  Persia, 
  were 
  made 
  over 
  to 
  me 
  for 
  examination. 
  

   Selected 
  samples 
  of 
  these 
  were 
  sliced 
  and 
  studied 
  under 
  the 
  micro- 
  

   scope, 
  and 
  the 
  results 
  are 
  embodied 
  in 
  the 
  following 
  pages. 
  

  

  Andesites. 
  

  

  [The 
  numbers 
  are 
  those 
  of 
  the 
  slides 
  in 
  my 
  own 
  collection.] 
  

   No. 
  1424. 
  Augite-hornblende-andesite. 
  Sp.gr. 
  = 
  2*682 
  

  

  1438. 
  

  

  1464. 
  Hornblende-aiidesite 
  

  

  1425. 
  

  

  1426. 
  

  

  1430. 
  Mica-andesite 
  

  

  1443. 
  „ 
  

  

  1436. 
  Andesite 
  

  

  2727 
  

   2-645 
  

   2 
  715 
  

   2-625 
  

   2-549 
  

   2-597 
  

   2-645 
  

  

  The 
  andesites 
  enumerated 
  above 
  are 
  in 
  some 
  respects 
  very 
  

   peculiar 
  rocks, 
  of 
  a 
  type 
  not 
  commonly 
  met 
  with. 
  They 
  vary 
  in 
  

   colour 
  from 
  grey 
  to 
  almost 
  white. 
  Many 
  of 
  them 
  are 
  very 
  trachy 
  tic- 
  

   looking, 
  and 
  some 
  years 
  ago 
  would 
  probably 
  have 
  been 
  classed 
  as 
  

   trachytes. 
  As, 
  however, 
  the 
  felspar 
  of 
  which 
  they 
  are 
  built 
  up 
  is 
  

   a 
  plagioclase, 
  and 
  not 
  sanidine, 
  they 
  cannot 
  be 
  called 
  by 
  that 
  name. 
  

  

  The 
  specimens 
  grouped 
  together 
  in 
  the 
  above 
  list 
  possess 
  several 
  

   features 
  in 
  common. 
  

  

  The 
  first 
  point 
  to 
  be 
  noted 
  is 
  their 
  low 
  specific 
  gravity, 
  which 
  

   averages 
  2*648. 
  The 
  mean 
  specific 
  gravity 
  of 
  quartzless 
  andesites, 
  

   according 
  to 
  text-books, 
  ranges 
  from 
  2*7 
  to 
  2*8 
  ; 
  so 
  that 
  the 
  average 
  

   densitj 
  r 
  of 
  the 
  rocks 
  now 
  described 
  is 
  somewhat 
  low. 
  The 
  percentage 
  

   of 
  silica 
  is 
  evidently 
  high, 
  but 
  I 
  have 
  not 
  observed 
  any 
  free 
  quartz 
  

   in 
  any 
  of 
  my 
  slides. 
  

  

  The 
  ordinary 
  method 
  of 
  determining 
  the 
  specific 
  gravity 
  of 
  a 
  rock 
  

   cannot 
  be 
  applied 
  to 
  these 
  specimens, 
  in 
  consequence 
  of 
  their 
  porosity. 
  

   I 
  boiled 
  the 
  samples 
  for 
  some 
  time 
  and 
  allowed 
  them 
  to 
  soak 
  in 
  

   water 
  from 
  24 
  to 
  48 
  hours 
  before 
  weighing 
  them 
  in 
  fresh 
  unboiled 
  

  

  