﻿GEOLOGY. 
  279 
  

  

  tainly 
  unaccountable. 
  The 
  Polyperfers^ 
  for 
  instance, 
  are, 
  

   when 
  in 
  tPieir 
  live 
  state, 
  of 
  a 
  calcareous 
  nature, 
  yai 
  we 
  

   find 
  them 
  now 
  in 
  the 
  strata 
  of 
  our 
  limestones 
  changed 
  

   into 
  silex." 
  And 
  in 
  this, 
  he 
  further 
  observes, 
  " 
  They 
  

   differ 
  from 
  the 
  organic 
  substances 
  of 
  other 
  countries." 
  

  

  Many 
  of 
  these 
  organic 
  remains, 
  supposed 
  to 
  have 
  been 
  

   derived 
  from 
  the 
  silurian 
  rocks, 
  consist 
  of 
  the 
  stony 
  

   axis 
  or 
  skeletons 
  of 
  polypi, 
  comprising 
  the 
  petrified 
  

   corals, 
  which, 
  as 
  a 
  class, 
  have 
  been 
  generally 
  designated 
  

   as 
  madrepores, 
  h»y 
  the 
  misapplication 
  of 
  a 
  term 
  belonging 
  

   properly 
  to 
  a 
  single 
  genus, 
  and 
  include 
  the 
  favosites 
  

   astreas, 
  cyathophylla 
  catenepora, 
  &c. 
  of 
  many 
  species, 
  

   and 
  of 
  great 
  variety 
  in 
  the 
  size, 
  structure, 
  and 
  arrange- 
  

   ment 
  or 
  grouping 
  of 
  the 
  cells. 
  Some 
  of 
  the 
  favosites, 
  

   from 
  the 
  size 
  and 
  form 
  of 
  the 
  cells, 
  by 
  a 
  very 
  popular 
  

   fallacy 
  have 
  been 
  regarded 
  as 
  petrified 
  honeycomb. 
  

  

  These, 
  however 
  numerous, 
  are 
  not 
  all 
  the 
  organic 
  

   remains 
  which 
  enter 
  into 
  this 
  group. 
  

  

  The 
  trilobite, 
  which, 
  in 
  its 
  native 
  lime 
  rock, 
  is 
  gene- 
  

   rally 
  found 
  of 
  rather 
  delicate 
  or 
  fragile 
  texture, 
  is 
  here 
  

   met 
  with 
  occasionally, 
  either 
  detached, 
  or 
  its 
  .form 
  im- 
  

   pressed 
  or 
  wholly 
  incorporated 
  in 
  the 
  chert 
  or 
  jaspery 
  

   pebble, 
  more 
  durable 
  than 
  granite. 
  

  

  Specimens 
  belonging 
  to 
  the 
  genera 
  calymene 
  and 
  

   asaphus 
  have 
  been 
  recognized. 
  

  

  Gorgonia, 
  fenestrella, 
  stems 
  of 
  crinoidea 
  or 
  encrenites, 
  

   as 
  well 
  as 
  orthocera 
  and 
  cyathophyllum, 
  are 
  found 
  in 
  

   similar 
  condition, 
  all, 
  but 
  most 
  especially 
  the 
  corals, 
  par- 
  

   taking 
  more 
  or 
  less 
  of 
  the 
  mineral 
  character 
  of 
  the 
  

   associated 
  agates 
  and 
  chalcedony, 
  which, 
  as 
  has 
  before 
  

   been 
  stated, 
  abound 
  in 
  the 
  drift. 
  

  

  Some 
  of 
  these, 
  when 
  cut 
  and 
  polished 
  by 
  the 
  lapidary, 
  

   form 
  gems 
  of 
  much 
  beauty, 
  and 
  have 
  been 
  greatly 
  ad- 
  

   mired. 
  

  

  