﻿Report 
  of 
  the 
  State 
  Geologist, 
  697 
  

  

  The 
  surface 
  of 
  the 
  " 
  serpentine" 
  when 
  fresh, 
  is 
  very 
  dark 
  green 
  

   or 
  black, 
  more 
  or 
  less 
  mottled 
  with 
  white 
  spots, 
  and, 
  commonly, 
  

   highly 
  polished. 
  This 
  polishing 
  is 
  due 
  to 
  abundant 
  slickensides 
  

   which 
  are 
  developed 
  on 
  a 
  large 
  scale. 
  The 
  rock 
  separates 
  along 
  

   these 
  slickensides 
  giving 
  curved 
  surfaces 
  often 
  100, 
  or 
  more 
  

   square 
  feet 
  in 
  area, 
  and 
  with 
  a 
  wonderful 
  lustre. 
  Some 
  portions 
  

   of 
  the 
  rock, 
  particularly 
  where 
  abundantly 
  slickensided, 
  have 
  

   almost 
  precisely 
  the 
  appearance 
  of 
  serpentine, 
  accounting 
  for 
  

   Emmons' 
  determination. 
  But 
  those 
  parts 
  which 
  are 
  mottled 
  

   present 
  a 
  different 
  aspect, 
  and 
  often 
  strongly 
  resemble 
  a 
  

   porphyritic 
  rock 
  with 
  glassy 
  base. 
  Examination, 
  both 
  with 
  the 
  

   naked 
  eye 
  and 
  with 
  the 
  microscope, 
  shows 
  that 
  the 
  mottling 
  is 
  

   due 
  to 
  the 
  presence 
  of 
  abundant 
  angular 
  grains 
  of 
  granitic 
  quartz, 
  

   which 
  are 
  enclosed 
  in 
  a 
  green 
  aggregate 
  of 
  uncertain 
  nature. 
  

   From 
  such 
  specimens 
  it 
  would 
  be 
  impossible 
  to 
  get 
  any 
  clear 
  idea 
  

   as 
  to 
  the 
  original 
  character 
  of 
  the 
  rock, 
  and 
  it 
  is 
  of 
  such 
  material 
  

   that 
  nearly 
  the 
  whole 
  exposure 
  consists. 
  But 
  fortunately 
  there 
  

   are 
  two 
  or 
  three 
  points 
  in 
  the 
  pit 
  where, 
  over 
  a 
  limited 
  area, 
  the 
  

   rock 
  retains 
  enough 
  of 
  its 
  original 
  character 
  to 
  show 
  clearly 
  its 
  

   origin. 
  At 
  these 
  points 
  it 
  gradually 
  becomes 
  lighter 
  colored, 
  the 
  

   quartz 
  loses 
  its 
  angular, 
  fragmental 
  appearance, 
  and 
  instead 
  of 
  

   the 
  green, 
  indefinite 
  aggregate, 
  pink 
  and 
  gray 
  feldspar 
  appears. 
  

   The 
  change 
  becomes 
  more 
  marked 
  toward 
  the 
  center 
  of 
  the 
  areas 
  

   till 
  finally 
  a 
  coarse-grained 
  granite 
  is 
  reached, 
  between 
  which 
  and 
  

   the 
  "serpentine" 
  there 
  is 
  a 
  complete 
  and 
  gradual 
  transition. 
  

   This 
  granite 
  is, 
  indeed, 
  considerably 
  altered, 
  but 
  not 
  sufficiently 
  

   so'to 
  obscure 
  its 
  true 
  character. 
  Moreover, 
  on 
  making 
  inquiries 
  

   of 
  the 
  engineer 
  in 
  charge 
  of 
  the 
  mines, 
  as 
  to 
  what 
  underlies 
  the 
  

   ore 
  at 
  the 
  bottom 
  of 
  the 
  deepest 
  shaft, 
  he 
  produced 
  a 
  specimen 
  

   of 
  the 
  rock 
  which 
  proved 
  to 
  be 
  a 
  fairly 
  fresh 
  granite, 
  resembling 
  

   many 
  outcrops 
  in 
  the 
  region 
  and 
  precisely 
  like 
  that 
  found 
  in 
  the 
  

   " 
  serpentine," 
  only 
  less 
  altered. 
  Although 
  no 
  extended 
  micros- 
  

   copic 
  examination 
  has 
  yet 
  been 
  made, 
  enough 
  has 
  been 
  done 
  to 
  

   show 
  clearly 
  that 
  there 
  is, 
  as 
  indicated 
  in 
  the 
  field, 
  a 
  complete 
  

   and 
  gradual 
  transition 
  between 
  the 
  " 
  serpentine 
  " 
  and 
  the 
  granite. 
  

   Thus, 
  it 
  is 
  evident 
  that 
  the 
  so-called 
  serpentine 
  at 
  the 
  Old 
  Sterling 
  

   mine 
  is 
  a 
  highly 
  altered 
  form 
  of 
  a 
  granite 
  which 
  was 
  intruded 
  

   into 
  the 
  ore 
  or 
  into 
  some 
  other 
  rock 
  which 
  has 
  sinoe 
  been 
  replaced 
  

   88 
  

  

  