﻿4 
  PROFESSOR 
  FORBES 
  ON 
  THE 
  VOLCANIC 
  GEOLOGY 
  OF 
  THE 
  VIVARAIS. 
  

  

  three 
  or 
  four 
  pretty 
  distinct 
  ranges 
  which 
  enclose 
  the 
  basin, 
  and 
  which 
  contribute 
  

   to 
  injure 
  its 
  climate 
  ; 
  for 
  there 
  is 
  little 
  wood, 
  and 
  the 
  winds 
  from 
  every 
  quarter 
  

   sweep 
  unchecked 
  over 
  the 
  extensive 
  and 
  bare 
  plateaux 
  of 
  granite 
  and 
  basalt 
  of 
  

   which 
  it 
  is 
  chiefly 
  composed.* 
  On 
  the 
  2oth 
  May 
  I 
  was 
  exposed 
  to 
  a 
  piercing 
  wind, 
  

   having 
  a 
  temperature 
  of 
  36°, 
  with 
  frequent 
  showers 
  of 
  hail. 
  A 
  few 
  days 
  later 
  I 
  

   engaged 
  a 
  horse 
  to 
  carry 
  me 
  across 
  the 
  highest 
  part 
  of 
  the 
  range 
  of 
  the 
  Cevennes, 
  

   and 
  the 
  sources 
  of 
  the 
  Loire, 
  down 
  to 
  the 
  valleys 
  of 
  the 
  Ardeche. 
  My 
  kind 
  friend 
  

   M. 
  Bertrand 
  had 
  given 
  me 
  a 
  route 
  which 
  should 
  embrace 
  the 
  most 
  interesting 
  

   geological 
  points 
  ; 
  and 
  with 
  his 
  map 
  as 
  a 
  guide, 
  I 
  started 
  alone 
  with 
  a 
  pony, 
  to 
  

   sleep 
  at 
  the 
  foot 
  of 
  the 
  Mont 
  Mezenc, 
  at 
  a 
  village 
  bearing 
  the 
  unpromising 
  name 
  

   of 
  Fay-le-froid. 
  Though 
  it 
  was 
  the 
  last 
  day 
  but 
  one 
  of 
  May, 
  little 
  appearance 
  of 
  

   spring 
  was 
  visible 
  ; 
  indeed, 
  it 
  had 
  hardly 
  an 
  opportunity 
  of 
  making 
  any 
  impression 
  

   on 
  the 
  singularly 
  bare 
  and 
  rugged 
  features 
  of 
  a 
  country 
  nearly 
  destitute 
  of 
  trees, 
  

   and 
  often 
  covered 
  for 
  miles 
  with 
  brown 
  angular 
  fragments 
  of 
  basalt. 
  

  

  The 
  position 
  of 
  Fay-le-froid, 
  a 
  meagre 
  village 
  of 
  little 
  more 
  than 
  a 
  single 
  

   street, 
  is 
  somewhat 
  greener 
  ; 
  it 
  lies 
  upon 
  the 
  northern 
  slope 
  of 
  the 
  Mont 
  Mezenc, 
  

   and 
  near 
  the 
  church 
  is 
  a 
  bed 
  of 
  basalt, 
  containing 
  fragments 
  of 
  trachyte 
  and 
  

   granite, 
  with 
  abundance 
  of 
  olivine. 
  Early 
  next 
  morning 
  I 
  was 
  on 
  the 
  gentle 
  

   ascent 
  which 
  leads 
  in 
  two 
  hours 
  to 
  the 
  summit 
  of 
  the 
  Mont 
  Mezenc. 
  A 
  fog 
  pre- 
  

   vented 
  me 
  from 
  enjoying 
  the 
  view,f 
  but 
  it 
  afterwards 
  cleared 
  away 
  sufficiently 
  

   to 
  enable 
  me 
  to 
  examine 
  the 
  geological 
  section 
  below 
  the 
  point 
  called 
  La 
  Croix 
  

   de 
  Boutieres, 
  which 
  is 
  800 
  feet 
  vertically 
  lower 
  than 
  the 
  top 
  of 
  the 
  Mezenc, 
  and 
  

   is 
  situated 
  a 
  little 
  to 
  the 
  south 
  of 
  it. 
  Here 
  the 
  escarpment 
  to 
  the 
  south-east 
  forms 
  

   a 
  sort 
  of 
  imperfect 
  amphitheatre, 
  in 
  which 
  the 
  Salliouse 
  (a 
  rivulet 
  joining 
  the 
  

   Erieux, 
  a 
  tributary 
  of 
  the 
  Rhone) 
  takes 
  its 
  rise. 
  This 
  hollow, 
  sometimes 
  called 
  

   Le 
  Cirque 
  de 
  Clusels, 
  presents 
  a 
  section 
  which 
  has 
  justly 
  obtained 
  some 
  celebrity 
  

   amongst 
  geologists. 
  The 
  peculiarity 
  which 
  it 
  presents 
  is 
  the 
  undoubted 
  super- 
  

   position 
  of 
  trachyte 
  and 
  phonolite 
  or 
  clinkstone 
  (which 
  are 
  felspathic 
  lavas), 
  to 
  

   common 
  basalt 
  and 
  vesicular 
  scoriae. 
  The 
  section 
  which 
  I 
  obtained, 
  and 
  from 
  

   which 
  I 
  took 
  specimens, 
  is 
  shewn 
  in 
  Plate 
  II., 
  fig. 
  1. 
  There 
  is 
  no 
  proper 
  super- 
  

   position 
  of 
  trachyte 
  to 
  phonolite 
  ; 
  the 
  latter 
  appears 
  to 
  pass 
  into 
  the 
  former, 
  and 
  

   sometimes 
  to 
  form 
  veins 
  in 
  it. 
  I 
  have 
  no 
  doubt 
  that 
  the 
  trachyte 
  is 
  scorified 
  and 
  

   rendered 
  vesicular 
  and 
  ochrey 
  by 
  the 
  heat 
  of 
  the 
  basalt 
  injected 
  from 
  below. 
  This 
  

   appears 
  to 
  be 
  the 
  utmost 
  which 
  can 
  be 
  legitimately 
  inferred 
  from 
  this 
  section 
  ; 
  and 
  

   it 
  is 
  so 
  far 
  a 
  satisfactory 
  conclusion, 
  since 
  it 
  does 
  not 
  contradict 
  the 
  generally 
  

   established 
  view 
  of 
  the 
  posteriority 
  of 
  basaltic 
  to 
  felspathic 
  lavas, 
  which 
  rule 
  be- 
  

   sides 
  receives, 
  in 
  this 
  immediate 
  neighbourhood, 
  so 
  unquestionable 
  a 
  support 
  from 
  

  

  * 
  See 
  Mr 
  Scrope's 
  Panorama 
  from 
  the 
  Montagne 
  d'Ours. 
  

  

  -f- 
  In 
  clear 
  weather 
  Mont 
  Blanc 
  is 
  visible 
  from 
  hence. 
  Bertrand, 
  Description 
  du 
  Puy, 
  8fc, 
  

   p. 
  124. 
  

  

  