﻿Radioactivity 
  of 
  certain 
  Lavas, 
  585 
  

  

  of 
  Vesuvius 
  as 
  an 
  active 
  volcano, 
  and 
  even 
  assisted 
  in 
  main- 
  

   taining 
  the 
  vulcanicity 
  in 
  the 
  surrounding 
  region. 
  There 
  

   may 
  be 
  other 
  cases 
  of 
  the 
  kind. 
  We 
  have 
  only 
  to 
  assume 
  

   that 
  here 
  and 
  there 
  in 
  the 
  deeper 
  crust 
  are 
  rock-masses 
  of 
  

   higher 
  radioactivity 
  -than 
  generally 
  prevails, 
  and 
  that 
  while 
  

   regional 
  vulcanicity 
  may 
  require 
  for 
  its 
  initiation 
  and 
  de- 
  

   velopment 
  prolonged 
  and 
  deep 
  sedimentation, 
  yet 
  where 
  

   there 
  is 
  coincidence 
  with 
  strongly 
  radioactive 
  magmas, 
  spe- 
  

   cially 
  energetic 
  and 
  persistent 
  effects 
  will 
  arise. 
  We 
  are, 
  of 
  

   course, 
  without 
  guidance 
  as 
  to 
  the 
  downward 
  extension 
  of 
  

   the 
  radioactive 
  laccolith 
  concerned 
  ; 
  or 
  as 
  to 
  whether 
  we 
  

   have 
  yet 
  attained 
  a 
  true 
  measure 
  of 
  its 
  radium-content. 
  

  

  An 
  explanation 
  of 
  the 
  exceptional 
  radioactivity 
  of 
  Vesuvian 
  

   lavas 
  may, 
  however, 
  also 
  be 
  sought 
  on 
  the 
  hypothesis 
  that 
  the 
  

   materials 
  ejected 
  from 
  Vesuvius 
  are 
  sedimentary 
  in 
  origin. 
  

   Such 
  an 
  origin 
  has 
  before 
  now 
  been 
  ascribed 
  to 
  volcanic 
  

   materials. 
  On 
  this 
  view 
  the 
  richness 
  in 
  radium 
  observed 
  

   in 
  these 
  lavas 
  is 
  in 
  line 
  with 
  the 
  observed 
  radium-richness 
  

   of 
  many 
  slow-collecting 
  sediments. 
  There 
  is, 
  indeed, 
  nothing 
  

   in 
  the 
  general 
  composition 
  of 
  these 
  lavas 
  to 
  negative 
  their 
  

   sedimentary 
  origin. 
  They 
  might 
  represent 
  certain 
  calcareous 
  

   clay 
  slates, 
  for 
  instance 
  *. 
  It 
  is 
  remarkable 
  that 
  the 
  

   percentage 
  of 
  potash 
  much 
  exceeds 
  that 
  of 
  soda. 
  Although 
  

   certain, 
  rather 
  scarce 
  igneous 
  rocks 
  — 
  the 
  shonkinites 
  and 
  

   some 
  others 
  — 
  sometimes 
  exhibit 
  a 
  similar 
  excess 
  of 
  potash 
  

   over 
  soda, 
  this 
  feature 
  is, 
  generally 
  speaking, 
  characteristic 
  

   of 
  detrital 
  remains 
  which 
  have 
  undergone 
  the 
  impoverishment 
  

   in 
  sodium 
  attendant 
  upon 
  denudation. 
  The 
  absolute 
  amounts 
  

   of 
  alkalies 
  present 
  are_, 
  however, 
  higher 
  than 
  is 
  commonly 
  

   found 
  in 
  sedimentary 
  rocks. 
  On 
  the 
  whole 
  this 
  view 
  seems 
  

   less 
  probable 
  than 
  that 
  which 
  assumes 
  the 
  Vesuvian 
  lavas 
  to 
  

   be 
  drawn 
  from 
  a 
  deep-seated 
  laccolith. 
  

  

  Whichever 
  explanation 
  of 
  the 
  origin 
  of 
  these 
  lavas 
  is 
  deemed 
  

   most 
  probable, 
  it 
  may 
  fairly 
  be 
  asked 
  if 
  we 
  are 
  indeed 
  entitled 
  

   to 
  assume 
  that 
  any 
  considerable 
  temperature-rise 
  may 
  be 
  

   ascribed 
  to 
  their 
  radioactivity. 
  This 
  question 
  discloses 
  the 
  

   unsatisfactory 
  state 
  of 
  our 
  knowledge 
  upon 
  the 
  subject. 
  All 
  

   depends 
  of 
  course 
  on 
  the 
  extension 
  of 
  the 
  radioactive 
  materials. 
  

   If 
  they 
  are 
  laccolithic 
  in 
  origin 
  we 
  may 
  reasonably 
  suppose 
  

   their 
  downward 
  extension 
  considerable, 
  and 
  in 
  this 
  case, 
  the 
  

   intensification 
  of 
  volcanic 
  conditions, 
  by 
  cumulative 
  radio- 
  

   active 
  energy, 
  appears 
  highly 
  probable. 
  If, 
  on 
  the 
  other 
  

   hand, 
  the 
  lavas 
  are 
  sedimentary 
  in 
  origin, 
  the 
  vertical 
  

   extension 
  of 
  beds 
  so 
  radioactive 
  can 
  hardly 
  be 
  great. 
  In 
  

  

  * 
  For 
  analyses 
  of 
  the 
  Vesuvian 
  lavas, 
  see 
  Roseubusch, 
  he. 
  cit. 
  p. 
  346. 
  

   PliiL 
  Mag. 
  S. 
  6. 
  Vol. 
  18. 
  No. 
  lOG. 
  Oct. 
  1909. 
  2 
  R 
  

  

  