﻿Vol. 
  53.] 
  ROCKS 
  AND 
  FOSSILS 
  FROM 
  FRANZ 
  JOSEF 
  LAND. 
  483 
  

  

  termed 
  ' 
  porphyritic 
  basalts,' 
  but 
  the 
  porphyritic 
  structure 
  is 
  never 
  

   strongly 
  marked 
  in 
  tbe 
  band-specimens, 
  and 
  is 
  frequently 
  not 
  

   noticeable. 
  

  

  Yesicular 
  and 
  amygdaloid 
  al 
  rocks 
  are 
  extremely 
  common 
  at 
  Cape 
  

   Flora. 
  This 
  is 
  a 
  point 
  of 
  some 
  interest, 
  when 
  considered 
  in 
  con- 
  

   nexion 
  with 
  Payer's 
  remark 
  that 
  ' 
  amygdaloidal 
  varieties, 
  so 
  

   common 
  in 
  Greenland, 
  were 
  never 
  found 
  in 
  Franz 
  Josef 
  Land.' 
  l 
  

   The 
  cavities 
  have 
  been 
  filled 
  with 
  various 
  substances, 
  such 
  as 
  

   calcite, 
  analcime, 
  natrolite, 
  chacedony, 
  quartz, 
  and 
  palagonite. 
  

  

  Under 
  the 
  microscope 
  the 
  constituents 
  are 
  seen 
  to 
  be 
  plagioclase, 
  

   augite, 
  magnetite, 
  olivine, 
  interstitial 
  matter, 
  and 
  various 
  secondary 
  

   products. 
  The 
  plagioclase 
  occurs 
  in 
  forms 
  giving 
  lath-shaped 
  

   sections, 
  and 
  also 
  as 
  aggregates 
  of 
  somewhat 
  larger 
  individuals, 
  

   which 
  mutually 
  interfere 
  with 
  each 
  other, 
  and 
  arc 
  more 
  equally 
  

   developed 
  in 
  the 
  different 
  directions. 
  A 
  broad 
  type 
  of 
  albite- 
  

   lamellation 
  is 
  common 
  to 
  both 
  modes 
  of 
  occurrence, 
  and 
  the 
  indi- 
  

   viduals 
  of 
  the 
  larger 
  aggregates 
  often 
  show, 
  in 
  addition, 
  a 
  zonal 
  

   structure 
  and 
  twinning 
  on 
  the 
  pericline-plan. 
  The 
  lath-shaped 
  

   sections 
  in 
  a 
  common 
  type 
  of 
  rock 
  measure 
  about 
  \5 
  mm. 
  in 
  length 
  

   by 
  '1 
  mm. 
  in 
  breadth; 
  whereas 
  the 
  individuals 
  which 
  compose 
  the 
  

   larger 
  aggregates 
  may 
  measure 
  as 
  much 
  as 
  1 
  or 
  2 
  mm. 
  in 
  their 
  

   longest 
  diameters. 
  There 
  is 
  a 
  certain 
  amount 
  of 
  variation 
  in 
  the 
  

   dimensions 
  of 
  the 
  felspars 
  in 
  different 
  specimens, 
  but 
  the 
  above 
  

   figures 
  will 
  give 
  an 
  idea 
  of 
  the 
  scale 
  on 
  which 
  they 
  are 
  commonly 
  

   developed 
  in 
  those 
  varieties 
  which 
  contain 
  comparatively 
  little 
  

   interstitial 
  matter. 
  

  

  When 
  the 
  powder 
  of 
  the 
  rock, 
  freed 
  from 
  the 
  fine 
  dust 
  by 
  

   washing, 
  is 
  placed 
  in 
  a 
  diffusion 
  -column 
  of 
  cadmium 
  borotungstate, 
  

   the 
  felspars 
  form 
  a 
  fairly 
  well-defined 
  band, 
  the 
  centre 
  of 
  which 
  

   corresponds 
  to 
  a 
  specific 
  gravity 
  of 
  2*7. 
  There 
  is 
  no 
  great 
  amount 
  

   of 
  scattering, 
  and 
  a 
  fragment 
  of 
  labradorite 
  floats 
  in 
  the 
  centre 
  of 
  

   the 
  band. 
  The 
  felspar, 
  therefore, 
  agrees 
  on 
  the 
  average 
  with 
  

   labradorite, 
  but 
  the- 
  optical 
  characters 
  of 
  the 
  zoned 
  individuals, 
  

   and 
  the 
  slight 
  scattering 
  of 
  the 
  grains 
  in 
  the 
  diffusion-column, 
  

   indicate 
  deviations 
  on 
  both 
  sides 
  of 
  the 
  average. 
  The 
  central 
  

   portions 
  of 
  the 
  zoned 
  individuals 
  are 
  more 
  basic 
  than 
  the 
  marginal 
  

   portions, 
  but 
  the 
  transition 
  is 
  not 
  always 
  continuous; 
  so 
  that 
  in 
  

   the 
  life-history 
  of 
  individual 
  crystals 
  there 
  has 
  occasionally 
  been 
  a 
  

   recurrence 
  of 
  the 
  conditions 
  which 
  gave 
  rise 
  to 
  the 
  deposition 
  of 
  

   more 
  basic 
  material. 
  The 
  larger 
  individuals 
  frequently 
  contain 
  

   inclusions 
  of 
  brown 
  glass, 
  with 
  or 
  without 
  bubbles. 
  These 
  inclusions 
  

   are 
  as 
  a 
  rule 
  limited 
  to 
  the 
  central 
  portions. 
  

  

  Augite 
  is 
  abundant 
  in 
  all 
  the 
  rocks, 
  and 
  forms, 
  with 
  felspar, 
  the 
  

   greater 
  portion 
  of 
  the 
  mass 
  in 
  the 
  majority 
  of 
  cases. 
  Generally 
  

   only 
  one 
  type 
  of 
  augite 
  is 
  present. 
  In 
  thin 
  sections 
  this 
  is 
  pale 
  

   brown, 
  more 
  rarely 
  yellowish 
  green, 
  sometimes 
  almost 
  colourless. 
  

   It 
  is 
  usually 
  without 
  any 
  trace 
  of 
  crystalline 
  form, 
  aud 
  occurs 
  as 
  

   grains 
  or 
  patches, 
  which 
  are 
  often 
  penetrated 
  by 
  the 
  lath-shaped 
  

   sections 
  of 
  plagioclase. 
  As 
  a 
  rule, 
  several 
  individuals 
  having 
  

   1 
  ' 
  New 
  Lands 
  within 
  the 
  Arctic 
  Circle,' 
  German 
  ed. 
  p. 
  267. 
  

  

  2x2 
  

  

  