158 
MISS  C.  A.  RAISIN  ON  VARIOLITE  OF 
[May  1893, 
The  variolite  is  also  similar  in  many  respects  to  pyromeride- 
bearing  rocks.  Among  the  differences,  we  find  that  the  more  basic 
sphernlites  do  not  tend  to  become  so  large,  the  usual  maximum  in 
the  Lleyn  examples  being  about  \  inch.  The  pyromerides  also  are 
generally  distinguished  by  a  smaller  proportion  of  ferrite,  and  by 
clearer  crystallization,  due  to  the  different  chemical  composition. 
As  to  form,  the  variole,  although  sometimes  nearly  spherical,  has 
an  elliptical  outline  more  commonly  than  the  acid  sphernlites,  and 
this  (and  other  characteristics)  may  be  connected  possibly  with  the 
more  fluid  character  of  a  basic  magma.  In  some  examples  veins 
and  circular  cracks  occur,1  similar  to  those  seen  at  the  exterior  of 
pyromerides  ;  but  here  the  mode  of  formation  is  clear  and  indis¬ 
putable,  since  the  material  of  the  spherulite  is  sharply  different 
from  that  of  the  vein.  In  the  more  acid  rock,  the  greater  similarity 
of  the  two  gives  an  appearance  of  gradation,  which  may  have 
seemed  to  lend  support  to  another  inference. 
VI.  Pseudocrystallites.2 
Microscopic  examination  reveals  occasionally  a  clear  network 
across  spherulites  or  other  parts  of  the  greenish  diabase.  The  bars 
of  the  net  appear  under  a  high  power  to  be  free  from  minute 
augite  and  epidote,  and  to  be  formed  of  felspathic  granules,  thus 
presenting  some  similarity  to  spherulitic  rays.  In  the  ferruginous 
variolite,  bars  of  granular  iron  oxide  are  similarly  developed, 
bordered  by  a  clear  isotropic  substance,  and  where  they  cross  the 
rays,  these  (for  a  short  distance)  are  free  from  the  brown  deposit. 
This  colouring  material  might  have  been  withdrawn  to  any  system 
of  contraction-cracks  if  such  existed ;  or  in  the  crystallization  of 
the  felspathic  constituent,  iron  oxide  would  be  extruded,  as  occurs 
along  spherulitic  rays.  Ferrite  is  aggregated  around  other  crystals, 
probably  pyroxene,  which  in  one  slide  vary  from  a  square  to  an 
oblong  or  lath-shape,  sometimes  so  narrow  that  the  dark  border 
becomes  a  bar ;  and  in  other  cases  feathery  tufts  grow  out  on  either 
side  of  the  pseudocrystallite-axis.  I  am  thus  inclined  to  revert 
to  the  earlier  explanation  of  these  structures,  and  not  to  follow 
Mr.  J.  W.  Gregory’s  idea  of  their  being  caused  by  contraction. 
VII.  Secondary  Minerals. 
Among  the  secondary  minerals  are  the  usual  aggregates  replacing 
felspar.  Other  crystals,  probably  originally  pyroxene,  enclose 
serpentinous,  chloritic,  or  similar  products,  and  are  surrounded  and 
crossed  by  deposits  of  iron  oxide.  A  similar  border  of  opacite  is 
seen  in  two  slides  of  trachyte  from  Auvergne,  which  were  kindly 
1  See  Mr.  Iddings’s  description  of  a  crescent-shaped  area  filled  with  felspar, 
in  a  spherulite  from  the  Yellowstone  Park,  Bull.  Phil.  Soc.  Washington, 
vol.  xi.  pp.  453,  454,  pi.  vii.  fig.  4. 
2  Cole  and  Gregory,  Quart.  Journ.  Geol.  Soc.  vol.  xlvi.  (1890)  pp.  313, 
314  ;  J.  W.  Gregory,  ibid.  vol.  xlvii.  (1891)  pp.  56-58;  Michel-Levy,  Bull.  Soc. 
Geol.  France,  3me  ser.  vol.  v.  (1877)  p.  238. 
