﻿208 
  GEOLOGY. 
  

  

  the 
  earth's 
  centre, 
  of 
  which, 
  practicallj^, 
  we 
  know 
  

   nothing, 
  the 
  investigations 
  of 
  geologists 
  as 
  to 
  its 
  exterior 
  

   coating 
  or 
  crust, 
  so 
  far 
  as 
  it 
  can 
  be 
  explored, 
  have 
  re- 
  

   sulted 
  in 
  establishing, 
  out 
  of 
  several 
  different 
  systems 
  

   that 
  have 
  been 
  proposed, 
  certain 
  principal 
  divisions, 
  

   separated 
  by 
  such 
  striking 
  differences 
  of 
  character 
  as 
  to 
  

   be 
  easily 
  distinguished, 
  and 
  which 
  for 
  convenience 
  it 
  

   has 
  been 
  found 
  desirable 
  to 
  adopt. 
  

  

  These 
  are 
  composed 
  of 
  groups 
  of 
  allied 
  or 
  analogous 
  

   strata, 
  supposed 
  to 
  have 
  been 
  produced 
  under 
  similar 
  

   circumstances 
  and 
  about 
  the 
  same 
  epoch, 
  and 
  are 
  desig- 
  

   nated 
  in 
  general 
  terms, 
  as 
  the 
  Primary, 
  the 
  Secondary, 
  

   and 
  the 
  Tertiary 
  formations 
  ; 
  and, 
  although 
  they 
  have 
  

   become 
  less 
  adequate 
  to 
  represent 
  the 
  present 
  state 
  of 
  

   geological 
  knowledge, 
  are 
  sufficiently 
  definite 
  for 
  the 
  

   present 
  purpose. 
  

  

  The 
  Primary 
  is 
  composed 
  of 
  the 
  primitive, 
  earliest 
  

   existent, 
  or 
  hypogene 
  rocks, 
  so 
  called, 
  as 
  being 
  formed 
  

   most 
  remote 
  from 
  the 
  surface. 
  

  

  They 
  are 
  of 
  a 
  crystalline 
  structure, 
  and 
  are 
  referred 
  

   to 
  an 
  igneous 
  origin, 
  and, 
  so 
  far 
  as 
  known, 
  are 
  desti- 
  

   tute 
  of 
  organic 
  remains, 
  indicating 
  a 
  commencement 
  

   anterior 
  to 
  that 
  of 
  animal 
  or 
  vegetable 
  existence. 
  

  

  The 
  Secondary 
  succeeds 
  in 
  the 
  ascending 
  order, 
  and 
  

   is 
  composed 
  of 
  marine 
  and 
  fresh-water 
  deposits. 
  Arena- 
  

   ceous, 
  argillaceous, 
  and 
  calcareous 
  rocks 
  form 
  the 
  prin- 
  

   cipal 
  masses, 
  and 
  are 
  associated 
  with 
  beds 
  of 
  chert, 
  

   ironstone, 
  and 
  coal. 
  The 
  strata 
  of 
  this 
  formation 
  are 
  

   characterized 
  by 
  innumerable 
  organic 
  remains 
  of 
  fish, 
  

   mollusca, 
  Crustacea, 
  &c., 
  belonging 
  wholly 
  to 
  extinct 
  

   species, 
  and 
  especially 
  by 
  numerous 
  and 
  gigantic 
  forms 
  

   of 
  saurian 
  reptiles, 
  indicating 
  that 
  the 
  portion 
  of 
  the 
  

   earth 
  which, 
  during 
  this 
  period, 
  had 
  emerged 
  above 
  the 
  

   surfiice 
  of 
  the 
  waters, 
  was 
  too 
  frequently 
  subject 
  to 
  in- 
  

  

  