﻿EROSION 
  OF 
  LAVA 
  — 
  CRATER 
  OF 
  JAUJAC. 
  15 
  

  

  jects, 
  as 
  in 
  Plate 
  II., 
  fig. 
  6, 
  and 
  gravity 
  thus 
  assists 
  the 
  atmospheric 
  causes 
  of 
  

   disintegration. 
  

  

  To 
  these 
  I 
  would 
  add 
  one 
  consideration 
  as 
  to 
  the 
  commencement 
  of 
  erosion. 
  

   When 
  the 
  form 
  of 
  a 
  lava-bed 
  confines 
  the 
  running 
  water 
  to 
  the 
  centre 
  of 
  a 
  stream, 
  

   as 
  in 
  fig. 
  7, 
  we 
  find 
  that 
  it 
  acts 
  extremely 
  slowly 
  ;* 
  but 
  if 
  the 
  river 
  takes 
  one 
  side 
  

   of 
  a 
  lava-bed, 
  as 
  in 
  fig. 
  8, 
  there 
  being 
  a 
  crevice 
  between 
  the 
  granite 
  and 
  the 
  lava, 
  

   the 
  water 
  must 
  penetrate, 
  and, 
  by 
  its 
  pressure, 
  tend 
  to 
  separate 
  the 
  columns, 
  and 
  

   to 
  wash 
  them 
  out, 
  so 
  that 
  I 
  know 
  of 
  no 
  existing 
  case 
  of 
  water 
  running 
  under 
  the 
  

   condition 
  last 
  described. 
  But 
  portions 
  of 
  the 
  lava 
  often 
  adhere 
  to 
  the 
  granite 
  

   even 
  on 
  the 
  side 
  of 
  the 
  valley 
  most 
  eroded. 
  This 
  is 
  the 
  case 
  on 
  the 
  bank 
  of 
  the 
  

   Alignon. 
  

  

  The 
  often 
  excessive 
  fragility 
  of 
  the 
  lava 
  also 
  assists 
  its 
  division 
  and 
  removal. 
  

   At 
  the 
  eboulement 
  of 
  Jaujac 
  the 
  basalt 
  is 
  singularly 
  brittle, 
  almost 
  the 
  whole 
  

   fallen 
  mass 
  is 
  shattered 
  into 
  bits 
  of 
  a 
  few 
  pounds 
  weight. 
  In 
  texture 
  it 
  

   slightly 
  resembles 
  pitch-stone, 
  and 
  the 
  lustre 
  is 
  that 
  of 
  animal 
  glue. 
  The 
  frag- 
  

   ments 
  include 
  many 
  pieces 
  of 
  granite. 
  At 
  the 
  junction 
  of 
  the 
  Alignon 
  and 
  Rio- 
  

   clat 
  is 
  a 
  considerable 
  mass 
  of 
  pure 
  feldspathic 
  granite, 
  not 
  like 
  that 
  of 
  the 
  country, 
  

   surrounded 
  by, 
  and 
  cased 
  in 
  basaltic 
  columns, 
  which 
  have 
  formed 
  almost 
  as 
  re- 
  

   gularly 
  as 
  if 
  it 
  had 
  not 
  been 
  there. 
  This, 
  though 
  very 
  interesting, 
  is 
  conformable 
  

   to 
  the 
  illustration 
  which 
  we 
  gave 
  of 
  the 
  law 
  of 
  direction 
  of 
  the 
  columns. 
  If 
  it 
  

   depended 
  upon 
  the 
  contact 
  of 
  the 
  lava 
  with 
  the 
  granite 
  otherwise 
  than 
  as 
  the 
  

   cooling 
  is 
  thereby 
  affected, 
  the 
  columns 
  might 
  be 
  expected 
  to 
  radiate 
  from 
  the 
  

   enclosed 
  mass; 
  but 
  as 
  any 
  mass, 
  not 
  enormous, 
  enveloped 
  in 
  such 
  a 
  stream, 
  

   might 
  acquire 
  the 
  temperature 
  of 
  the 
  melted 
  matter, 
  the 
  whole 
  would 
  cool 
  with 
  

   reference 
  only 
  to 
  the 
  conditions 
  extraneous 
  to 
  it. 
  The 
  lava 
  extends 
  but 
  little 
  

   way 
  up 
  the 
  Rioclat, 
  which, 
  however, 
  presents 
  a 
  curious 
  deposit 
  of 
  volcanic 
  ashes, 
  

   which 
  no 
  doubt 
  must 
  have 
  fallen 
  from 
  the 
  crater 
  of 
  Jaujac. 
  

  

  The 
  Coupe 
  de 
  Jaujac 
  (being 
  an 
  exact 
  translation 
  of 
  the 
  word 
  crater, 
  by 
  

   which 
  the 
  ancients 
  denoted 
  a 
  volcanic 
  orifice) 
  is 
  distant 
  only 
  half-a-mile 
  or 
  a 
  

   mile 
  from 
  the 
  village, 
  in 
  a 
  natural 
  opening 
  or 
  cavity 
  between 
  two 
  primitive 
  

   mountains, 
  filled 
  with 
  coal-formation 
  sandstone, 
  whose 
  character 
  is 
  well 
  marked 
  

   by 
  abundant 
  impressions 
  of 
  fossil 
  plants. 
  This 
  formation 
  (in 
  which 
  coal 
  was 
  at 
  

   one 
  time 
  worked) 
  t 
  is 
  interesting 
  on 
  several 
  accounts, 
  although 
  the 
  occurrence 
  of 
  

   the 
  volcano 
  in 
  the 
  midst 
  of 
  it 
  cannot 
  but 
  be 
  regarded 
  as 
  entirely 
  accidental. 
  Its 
  

   extent 
  is 
  small, 
  being 
  nearly 
  confined 
  to 
  the 
  valley 
  of 
  Prades, 
  adjoining 
  that 
  of 
  

   the 
  Alignon, 
  at 
  least 
  if 
  the 
  Geological 
  Map 
  of 
  France 
  be 
  correct. 
  It 
  is, 
  as 
  already 
  

   observed, 
  part 
  of 
  a 
  widely-spread 
  but 
  often-interrupted 
  ring 
  of 
  the 
  coal-formation, 
  

   surrounding 
  the 
  primitive 
  plateau 
  of 
  Central 
  France. 
  I 
  examined, 
  very 
  carefully, 
  

  

  * 
  Examples 
  : 
  — 
  The 
  tributary 
  on 
  the 
  left 
  of 
  Ardeche 
  near 
  Thuez 
  ; 
  lava 
  of 
  Burzet 
  at 
  the 
  cascade 
  

   near 
  the 
  village. 
  

  

  "j" 
  On 
  the 
  authority 
  of 
  Faujas 
  St 
  Fond. 
  

  

  