﻿Vol. 
  53.] 
  PROF. 
  E. 
  EAYSER 
  ON 
  VOLCANIC 
  BOMBS 
  IN 
  NASSAU. 
  109 
  

  

  7. 
  Note 
  on 
  Volcanic 
  Bombs 
  in 
  the 
  Schalsteins 
  of 
  Nassau. 
  By 
  

   Prof. 
  E. 
  Kayser, 
  Ph.D., 
  For. 
  Corr. 
  G.S. 
  (Communicated 
  by 
  the 
  

   Secretary. 
  Eead 
  December 
  16th, 
  1896.) 
  

  

  The 
  origin 
  of 
  the 
  schalsteins 
  of 
  Nassau, 
  the 
  Harz, 
  and 
  other 
  districts 
  

   has 
  remained 
  hitherto 
  an 
  unsolved 
  enigma 
  for 
  geologists. 
  This, 
  

   however, 
  is 
  not 
  surprising, 
  considering 
  the 
  very 
  variable 
  nature 
  of 
  the 
  

   rocks. 
  Generally 
  of 
  greyish-green 
  colour, 
  with 
  highly-developed 
  slaty 
  

   cleavage, 
  and 
  with 
  a 
  large 
  admixture 
  of 
  calcareous 
  and 
  chloritic 
  

   matter, 
  the 
  schalsteins 
  are 
  sometimes 
  earthy, 
  sometimes 
  coarse- 
  

   grained 
  to 
  conglomeratic 
  and 
  brecciated 
  — 
  and 
  in 
  this 
  latter 
  case 
  

   they 
  contain 
  frequently 
  fragments 
  of 
  foreign 
  rocks 
  of 
  extremely 
  

   diverse 
  derivation, 
  such 
  as 
  diabase, 
  porphyry, 
  limestone, 
  slate, 
  etc. 
  

   The 
  schalsteins 
  are, 
  moreover, 
  sometimes 
  fossiliferous 
  and 
  distinctly 
  

   bedded, 
  and 
  sometimes 
  unfossiliferous, 
  with 
  as 
  little 
  evidence 
  of 
  

   stratification 
  as 
  an 
  eruptive 
  rock. 
  

  

  Some 
  geologists 
  consider 
  the 
  schalsteins 
  as 
  originating 
  from 
  a 
  

   mixture 
  of 
  disintegrated 
  diabases 
  and 
  ordinary 
  argillaceous 
  mud 
  ; 
  

   others 
  see 
  in 
  them 
  a 
  product 
  of 
  metamorphic 
  processes 
  ; 
  while 
  most 
  

   observers 
  look 
  upon 
  them 
  as 
  volcanic 
  tuffs, 
  but 
  greatly 
  changed 
  

   in 
  consequence 
  of 
  their 
  great 
  age. 
  That 
  this 
  last 
  opinion 
  is 
  the 
  

   only 
  correct 
  one 
  seems 
  clearly 
  proved 
  by 
  the 
  bombs 
  which 
  I 
  have 
  

   discovered. 
  

  

  These 
  bombs 
  are 
  found 
  in 
  great 
  numbers 
  in 
  two 
  localities 
  in 
  the 
  

   neighbourhood 
  of 
  Oberscheld, 
  near 
  Dillenburg 
  : 
  firstly 
  in 
  the 
  vicinity 
  

   of 
  the 
  village 
  of 
  Bicken 
  (well 
  known 
  to 
  the 
  palaeontologist 
  on 
  

   account 
  of 
  the 
  occurrence 
  there 
  of 
  Hercynian 
  and 
  Upper 
  Devonian 
  

   fossils), 
  in 
  a 
  fine, 
  earthy 
  schalstein, 
  very 
  well 
  stratified 
  ; 
  and 
  

   secondly, 
  close 
  to 
  Oberscheld, 
  in 
  a 
  slaty 
  rock, 
  less 
  clearly 
  stratified. 
  

  

  In 
  the 
  first 
  locality 
  the 
  rock 
  forms 
  a 
  small 
  hill, 
  projecting 
  from 
  

   the 
  surrounding 
  Culm 
  Measures 
  and 
  constituting 
  geologically 
  an 
  

   anticlinal 
  fold 
  ; 
  in 
  the 
  second 
  locality 
  it 
  is 
  only 
  separated 
  from 
  the 
  

   superjacent 
  Culm 
  Measures 
  by 
  a 
  moderately 
  thick 
  layer 
  of 
  diabase. 
  

   The 
  above-mentioned 
  schalsteins 
  must, 
  therefore, 
  belong 
  to 
  the 
  

   uppermost 
  horizon 
  of 
  the 
  Upper 
  Devonian. 
  

  

  The 
  bombs 
  are 
  generally 
  rounded, 
  sometimes, 
  however, 
  angular 
  ; 
  

   varying 
  in 
  size 
  from 
  that 
  of 
  a 
  walnut 
  to 
  that 
  of 
  a 
  man's 
  fist, 
  but 
  

   sometimes 
  as 
  big 
  as 
  a 
  man's 
  head. 
  They 
  consist 
  of 
  a 
  kernel 
  of 
  

   coarse-grained 
  rock 
  resembling 
  gabbro, 
  with 
  a 
  comparatively 
  thin 
  

   rind 
  of 
  ' 
  mandelstein.' 
  The 
  kernel 
  is 
  composed, 
  according 
  to 
  the 
  

   researches 
  of 
  my 
  colleague, 
  Prof. 
  M. 
  Bauer, 
  of 
  a 
  crystalline 
  aggregate 
  

   of 
  calcite, 
  mica, 
  chlorite, 
  and 
  other 
  minerals, 
  and 
  represents 
  frag- 
  

   ments 
  of 
  limestone 
  altered 
  by 
  contact-metamorphism. 
  The 
  rind 
  is 
  

   1 
  to 
  2 
  centim. 
  thick, 
  generally 
  finely 
  porous, 
  but 
  contains 
  occasionally 
  

   very 
  large 
  bubble-cavities, 
  filled 
  with 
  calcite 
  or 
  zeolites 
  

  

  The 
  Oberscheld 
  bombs 
  are 
  therefore 
  very 
  similar 
  to 
  the 
  well- 
  

   known 
  bombs 
  of 
  olivine, 
  mica, 
  hornblende, 
  etc., 
  encrusted 
  with 
  a 
  

   rind 
  of 
  lava, 
  found 
  in 
  the 
  tuffs 
  of 
  the 
  Eifel, 
  Auvergne, 
  and 
  other 
  

   volcanic 
  districts. 
  

  

  