﻿GEOLOGY. 
  211 
  

  

  period 
  of 
  their 
  formation; 
  and 
  thus 
  it 
  is 
  that 
  all 
  minerals 
  

   are 
  not 
  to 
  be 
  found 
  or 
  expected 
  alike 
  in 
  all 
  situations, 
  or 
  

   where 
  the 
  strata 
  to 
  which 
  they 
  properly 
  belong 
  do 
  not 
  

   and 
  cannot 
  in 
  the 
  nature 
  of 
  things, 
  exist. 
  

  

  The 
  thickness 
  of 
  all 
  the 
  strata 
  taken 
  collectively, 
  as 
  

   estimated 
  on 
  the 
  section 
  before 
  referred 
  to, 
  Plate 
  IX., 
  

   amounts 
  to 
  several 
  miles. 
  Now, 
  as 
  the 
  deepest 
  shaft 
  

   yet 
  sunk 
  into 
  the 
  earth 
  in 
  mining 
  operations 
  is 
  said 
  not 
  

   greatly 
  to 
  exceed 
  half 
  a 
  mile, 
  it 
  may 
  be 
  asked 
  how 
  it 
  is 
  

   that 
  geologists 
  have 
  arrived 
  at 
  a 
  knowledge 
  of 
  so 
  much 
  

   of 
  the 
  earth's 
  crust 
  as 
  they 
  claim 
  to 
  have 
  acquired 
  ? 
  

  

  If 
  the 
  different 
  strata 
  had 
  remained 
  undisturbed 
  in 
  

   the 
  position 
  in 
  which 
  they 
  were 
  first 
  deposited, 
  we 
  must 
  

   have 
  remained 
  forever 
  ignorant, 
  not 
  only 
  of 
  the 
  character 
  

   and 
  thickness 
  of 
  these 
  strata, 
  but 
  of 
  their 
  very 
  exist- 
  

   ence. 
  But, 
  in 
  those 
  great 
  convulsions 
  which 
  the 
  earth 
  

   has 
  undergone, 
  in 
  the 
  upheaval 
  of 
  entire 
  and 
  lofty 
  

   mountain 
  ranges, 
  the 
  pre-existent 
  and 
  superincumbent 
  

   strata 
  have 
  been 
  tilted 
  up, 
  dislocated, 
  and 
  inclined 
  Avith 
  

   their 
  edges 
  to 
  the 
  surface, 
  as 
  is 
  represented 
  in 
  the 
  section 
  

   Plate 
  X., 
  Figure 
  1. 
  

  

  This 
  section 
  will 
  illustrate 
  the 
  fact 
  that, 
  in 
  travelling 
  

   over 
  a 
  country 
  that 
  has 
  been 
  so 
  convulsed, 
  across 
  the 
  

   strata, 
  or 
  in 
  a 
  direction 
  from 
  A 
  to 
  B, 
  the 
  different 
  strata 
  

   will 
  be 
  passed 
  over 
  successively 
  as 
  they 
  appear 
  upon 
  the 
  

   surface. 
  

  

  Now 
  suppose 
  that, 
  along 
  this 
  distance, 
  a 
  series 
  of 
  

   natural 
  sections, 
  such 
  as 
  the 
  channels 
  of 
  streams 
  or 
  

   ravines, 
  should 
  occur 
  at 
  different 
  intervals, 
  or 
  that 
  at 
  the 
  

   points 
  C, 
  D, 
  E, 
  wells 
  or 
  shafts 
  should 
  have 
  been 
  sunk, 
  

   these 
  several 
  strata 
  would 
  be 
  revealed, 
  and 
  the 
  dip 
  or 
  

   angle 
  of 
  inclination, 
  ascertained, 
  and 
  consequently 
  the 
  

   thickness 
  of 
  each 
  or 
  of 
  the 
  whole 
  from 
  B 
  to 
  F 
  might 
  be 
  

   readily 
  determined. 
  

  

  