﻿ACADEMY 
  OP 
  SCIENCES. 
  15 
  

  

  dition, 
  even 
  if 
  it 
  docs 
  for 
  the 
  second. 
  Granite, 
  in 
  the 
  Placer 
  county 
  gravel 
  

   beds, 
  occurs 
  only 
  in 
  boulders 
  associated 
  with 
  the 
  quartz, 
  and 
  that 
  sparingly, 
  the 
  

   bed 
  rock 
  being 
  universally 
  a 
  slate 
  ; 
  and 
  in 
  this 
  respect, 
  the 
  difference 
  between 
  

   the 
  placer 
  diggings 
  of 
  Idaho 
  Territory 
  and 
  those 
  of 
  central 
  California, 
  is 
  very 
  

   remarkable. 
  In 
  Idaho, 
  the 
  bed 
  rock 
  is 
  everywhere 
  granite 
  ; 
  and 
  the 
  ledges 
  

   which 
  have 
  supplied 
  the 
  gold 
  are 
  often 
  distinctly 
  traceable, 
  good 
  diggings 
  being 
  

   found 
  below 
  them, 
  as 
  in 
  Granite 
  Gulch, 
  near 
  Placerville, 
  and 
  none 
  at 
  all 
  above. 
  

  

  To 
  what, 
  then, 
  shall 
  we 
  refer 
  the 
  disruption, 
  in 
  California, 
  of 
  that 
  primitive 
  

   relationship 
  of 
  rocks, 
  which 
  we 
  find 
  still 
  remaining 
  in 
  Idaho? 
  

  

  Perhaps 
  volcanic 
  action 
  may 
  account 
  for 
  it 
  ; 
  and 
  in 
  connection 
  with 
  this 
  view 
  

   I 
  wish 
  to 
  present 
  to 
  the 
  notice 
  of 
  the 
  Society 
  a 
  specimen 
  of 
  the 
  peculiar 
  sub- 
  

   stance 
  called 
  cement. 
  This 
  substance 
  occurs 
  very 
  abundantly 
  in 
  distinct, 
  

   and 
  sometimes 
  alternate, 
  stratification 
  with 
  the 
  gravel, 
  in 
  most 
  of 
  the 
  Placer 
  

   county 
  mines 
  ; 
  in 
  fact, 
  in 
  all 
  of 
  them 
  which 
  I 
  had 
  opportunity 
  of 
  visiting. 
  It 
  

   does 
  not, 
  so 
  far 
  as 
  I 
  could 
  see, 
  mix 
  with 
  the 
  gravel, 
  but 
  is 
  often 
  of 
  a 
  depth 
  and 
  

   hardness 
  as 
  seriously 
  to 
  embarrass 
  the 
  operations 
  of 
  the 
  miner. 
  Being 
  entirely 
  

   barren, 
  it 
  has 
  sometimes 
  to 
  b^ 
  blasted 
  with 
  powder 
  or 
  nitro-glycerine, 
  before 
  the 
  

   hydraulic 
  stream 
  will 
  act 
  upon 
  it, 
  and 
  then 
  adds 
  greatly 
  to 
  the 
  cost 
  of 
  hydraulic 
  

   operations. 
  

  

  As 
  will 
  be 
  observed, 
  it 
  is 
  a 
  grayish 
  white, 
  and 
  so 
  homogeneous, 
  apparently, 
  

   in 
  its 
  nature, 
  that 
  the 
  miners 
  gt 
  nerally, 
  though 
  very 
  ignorantly, 
  call 
  it 
  pipe- 
  

   clay. 
  Although 
  this 
  whitish 
  color 
  is 
  the 
  usual 
  tint, 
  I 
  have 
  observed 
  it 
  in 
  some 
  

   situations 
  to 
  be 
  of 
  various 
  shades 
  of 
  brown. 
  

  

  Now, 
  is 
  this 
  substance 
  a 
  volcanic 
  ash, 
  and 
  if 
  not, 
  what 
  is 
  it? 
  

  

  I 
  think 
  the 
  answer 
  to 
  this 
  question 
  carries 
  with 
  it 
  a 
  solution 
  of 
  much 
  of 
  the 
  

   difficulty 
  in 
  accounting 
  for 
  the 
  condition 
  of 
  things 
  in 
  central 
  California. 
  Ad- 
  

   raittiug 
  that 
  this 
  cement 
  is 
  a 
  true 
  product 
  of 
  volcanic 
  eruption, 
  the 
  large 
  extent 
  

   of 
  surface 
  covered 
  by 
  it 
  and 
  its 
  frequent 
  great 
  depth, 
  would 
  lead 
  us 
  to 
  infer 
  an 
  

   enormous 
  amount 
  of 
  volcanic 
  activity, 
  perhaps 
  in 
  connection 
  with 
  the 
  elevation 
  

   of 
  the 
  neighboring 
  peaks 
  of 
  the 
  Sierra 
  Nevada 
  range. 
  

  

  Mr. 
  Hanks 
  kindly 
  afforded 
  me 
  a 
  microscopic 
  examination 
  of 
  the 
  present 
  speci- 
  

   men, 
  and 
  it 
  appears 
  to 
  resolve 
  itself 
  into 
  the 
  three 
  elements 
  of 
  granite 
  — 
  quartz, 
  

   mica 
  and 
  feldspar. 
  This 
  is 
  not 
  an 
  unusual 
  condition 
  of 
  volcanic 
  ash, 
  and 
  if 
  my 
  

   impression 
  is 
  correct, 
  it 
  is, 
  with 
  the 
  addition 
  of 
  sulphur, 
  exactly 
  the 
  analysis 
  of 
  

   the 
  ash 
  ejected 
  in 
  the 
  recent 
  eruption 
  of 
  Vesuvius. 
  

  

  But 
  cveu 
  considering 
  it 
  as 
  settled 
  that 
  cement 
  is 
  a 
  volcanic 
  ash, 
  solidified 
  by 
  

   time 
  and 
  pressure, 
  we 
  have 
  still 
  two 
  things 
  to 
  account 
  for 
  ; 
  one, 
  the 
  almost 
  

   total 
  disappearance 
  of 
  the 
  granite, 
  the 
  other 
  the 
  levigation 
  of 
  the 
  quarfz. 
  

  

  After 
  due 
  consideration 
  of 
  the 
  effects 
  of 
  prolonged 
  action 
  of 
  the 
  surf 
  on 
  both 
  

   salt 
  and 
  fresh 
  water 
  beaches, 
  in 
  the 
  production 
  of 
  such 
  gravel 
  and 
  boulders 
  as 
  

   we 
  see 
  in 
  Placer 
  county 
  — 
  as 
  I 
  doubt 
  whether 
  the 
  ancient 
  river 
  system 
  can 
  be 
  

   taken 
  into 
  the 
  question, 
  or 
  is 
  so 
  clearly 
  traceable 
  — 
  there 
  is 
  one 
  other 
  mode 
  of 
  

   explanation 
  of 
  most, 
  if 
  not 
  all 
  the 
  phenomena 
  alluded 
  to, 
  which 
  I 
  think 
  deserves 
  

   attention. 
  I 
  refer 
  to 
  the 
  grinding 
  and 
  comminuting 
  power 
  of 
  glacial 
  action. 
  

  

  Of 
  all 
  the 
  forces 
  of 
  nature 
  which 
  effect 
  transformation 
  of 
  the 
  surface 
  of 
  the 
  

  

  