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  NEW 
  YORK 
  STATE 
  MUSEUM 
  • 
  

  

  ScMstj 
  mica 
  scMst, 
  hydromica 
  ischist, 
  talcose 
  schist, 
  etc. 
  

   Various 
  members 
  of 
  the 
  mica 
  group 
  play 
  an 
  important 
  part 
  in 
  

   the 
  schists. 
  

  

  Slate, 
  This 
  is 
  mainly 
  shale 
  hardened 
  by 
  metamorphism 
  and 
  

   rendered 
  fissile 
  by 
  pressure. 
  The 
  roofing 
  slates 
  are 
  good 
  ex- 
  

   amples. 
  

  

  Crystalline 
  limestone. 
  This 
  is 
  sedimentary 
  limestone 
  made 
  

   crystalline 
  by 
  heat 
  and 
  pressure. 
  

  

  All 
  kinds 
  of 
  igneous 
  rocks 
  may 
  become 
  schistose 
  by 
  metamor- 
  

   phism 
  and 
  then 
  receive 
  names 
  indicating 
  their 
  composition 
  and 
  

   structure. 
  

  

  HISTORIC 
  GEOLOGY 
  

  

  Historic 
  geology 
  treats 
  of 
  the 
  succession 
  of 
  geologic 
  deposits 
  

   and 
  is 
  based 
  on 
  the 
  study 
  of 
  sedimentary 
  rocks. 
  

  

  It 
  is 
  estimated 
  that 
  the 
  geologic 
  series 
  consists 
  of 
  about 
  

   100,000 
  feet 
  or 
  20 
  miles 
  thickness 
  of 
  sedimentary 
  strata. 
  These 
  

   are 
  beds 
  of 
  sediment 
  chiefly 
  formed 
  by 
  successive 
  invasions 
  of 
  

   the 
  sea 
  and 
  the 
  transportation 
  and 
  deposition 
  by 
  It 
  of 
  debris 
  de- 
  

   tached 
  from 
  the 
  rocks 
  of 
  the 
  mainland 
  by 
  rain, 
  frost, 
  rivers 
  and 
  

   the 
  ocean 
  waves. 
  

  

  It 
  has 
  been 
  estimated 
  that 
  about 
  99^ 
  of 
  all 
  rocks 
  are 
  sedi- 
  

   mentary, 
  and 
  although 
  some 
  of 
  these 
  were 
  formed 
  in 
  fresh 
  

   water, 
  probably 
  the 
  larger 
  part 
  of 
  the 
  sedimentary 
  rocks 
  were 
  

   deposited 
  in 
  the 
  ocean. 
  It 
  has 
  consequently 
  been 
  said 
  that 
  

   * 
  the 
  sea 
  is 
  the 
  mother 
  of 
  continents.' 
  On 
  our 
  Atlantic 
  coast, 
  

   as 
  elsewhere, 
  the 
  ocean 
  is 
  both 
  a 
  destructive 
  and 
  a 
  formative 
  

   agent. 
  As 
  the 
  soundings 
  show, 
  the 
  loose 
  materials 
  washed 
  from 
  

   the 
  land 
  are 
  spread 
  out 
  about 
  100 
  miles 
  from 
  the 
  shore 
  line 
  

   in 
  a 
  broad, 
  sloping 
  plain 
  of 
  sand 
  and 
  mud. 
  In 
  such 
  submarine 
  

   deposits, 
  when 
  uncovered 
  by 
  the 
  ocean's 
  retreat, 
  we 
  find 
  the 
  

   remains 
  of 
  mollusks, 
  fishes 
  and 
  other 
  marine 
  forms 
  of 
  life. 
  Be- 
  

   sides, 
  land 
  animals 
  are 
  often 
  drowned 
  and 
  their 
  bodies 
  are 
  carried 
  

   out 
  to 
  sea 
  and 
  covered 
  with 
  sediment, 
  leaves 
  fall 
  on 
  the 
  water 
  

   and 
  sink 
  to 
  the 
  bottom. 
  Therefore, 
  in 
  rocks 
  formed 
  in 
  the 
  sea 
  

   we 
  sometimes 
  find 
  remains 
  of 
  land 
  animals 
  and 
  plants, 
  besides 
  

   the 
  marine 
  forms 
  which 
  we 
  expect. 
  The 
  unceasing 
  action 
  of 
  

  

  