156 
MISS  C.  A.  RAISIN  ON  VARIOLITE  OF 
[May  1893, 
basic  rocks,  are  bounded  either  by  mutual  planes  of  weakness  or  by 
an  intervening  magma  differently  consolidated.1  An  example  of  the 
first  case  is  the  Arran  felsite,  in  which  the  polygonal  boundaries,  as 
was  pointed  out  by  Prof.  Bonney,  were  probably  due  to  the  contrac¬ 
tion  which  followed  a  renewed  heating  of  the  rock.2 3  Where  spheru- 
lites  are  separated  by  intervening  palagonite,  this  is  likely  to  be  a 
modification  of  the  original  glass.  If  the  temperature  had  been  much 
raised  subsequently,  we  might  expect  some  marginal  devitrification 
and  the  development  perhaps  of  half-spherulites,  as  in  the  artificial 
example  described  by  Prof.  Bonney. !  But  the  modified  glass  which 
borders  cracks  forms  a  zone  outside  any  spherulitic  or  devitrified 
structure.  Thus,  although  in  a  spheroid  the  fissures  formed  by 
contraction  might  allow  the  passage  of  residual  heat,  causing  a 
secondary  devitrification  in  the  way  suggested,  by  Prof.  Cole  in 
Lipari  obsidian,4  I  think  that  in  these  examples  the  formation  of 
spherulites  was  generally  a  result  of  the  original  cooling.  Struc¬ 
tures  developed  in  the  two  ways  could  not  always  be  distinguished, 
but,  as  in  the  experiments  of  dry  cooling,5 6  it  would  seem  that 
both  conditions  occur. 
Considering  the  origin  of  acid  spherulites,  Mr.  Iddings  attributes 
the  crystallization  of  certain  portions  of  the  magma  to  their  being 
places  of  greater  hydration  ; G  Mr.  Whitman  Cross  to  the  aggrega¬ 
tion  of  a  colloid  substance.7  In  these  andesites  or  basalts  of  the 
Lleyn  we  note  frequently  that  spherulites  have  a  concentration  of 
iron  oxide  towards  the  centre,  and  since  this  substance  is  an  early 
deposit,  it  is  possible  that,  in  somewhat  basic  magmas,  its  segrega¬ 
tion  might  help  to  induce  the  formation  of  spherulites  at  certain 
spots.8 
As  original  constituents,  the  spherulites  seem  to  form  in  a  mass 
where  the  rate  of  cooling  is  not  uniform,  and  neither  slow  enough 
to  allow  of  the  development  of  well-formed  crystals,  nor  so  rapid  as 
to  give  rise  to  a  homogeneous  glass.  As  Prof.  Bonney  has  said,  the 
conditions  are  probably  found  in  a  mass  kept  for  some  time  at  a 
1  See  J.  P.  Iddings,  ‘  Spherulitic  Crystallization,’  Bull.  Phil.  Soc.  Wash¬ 
ington,  vol.  xi.  (1891)  p.  451. 
2  Geol.  Mag.  for  1877,  p.  510. 
3  Quart.  Journ.  Geol.  Soc.  vol.  xli.  (1885)  Proc.  p.  92. 
4  Min.  Mag.  vol.  ix.  (1891)  p.  274. 
5  Quart.  Journ.  Geol.  Soc.  vol.  xli.  (1885)  Proc.  p.  93. 
6  Bull.  Phil.  Soc.  Washington,  vol.  xi.  (1891)  p.  447. 
7  ‘  Constitution  and  Origin  of  Spherulites  in  Acid  Eruptive  Rocks,’  ibid. 
pp.  436,  437. 
8  The  iron  oxide  in  these  rocks  often  is  not  in  its  original  condition,  but 
although  oxidation  and  hydration  may  have  occurred,  it  seems  probable  that 
the  spherulites  are  marked  by  an  excess  of  the  ferruginous  constituent.  In  the 
well-known  pyromeride-bearing  felstone  of  Lea  Rock,  Shropshire,  the  iron  oxide 
which  causes  the  red-brown  colour  of  the  small  spherulites  may  be  only  the 
result  of  subsequent  change  ;  but  it  is  possible  that  a  smaller  proportion  of  iron 
is  present  in  the  surrounding  matrix,  in  which  no  large  amount  of  viridite  or 
of  any  iron-containing  constituent  can  be  recognized  on  microscopic  examina¬ 
tion.  It  would  seem  to  be  impossible  here  to  apply  the  final  test  of  chemical 
analysis.  Compare  also  some  of  the  pyromerides  of  Boulay  Bay,  Jersey. 
