190  PEOF.  J.  W.  JUDD  ON  INCLUSIONS  OF  TERTIARY  [May  1893, 
(1  inch)  in  diameter  have  been  formed,  and  then  a  complete  change 
in  the  mode  of  development  has  taken  place,  the  outer  zones  of  the 
spherulite  being  formed  by  the  separating  concentric  layers  of  a 
lithophyse  (see  Pl.  II.  fig.  4). 
Although  the  state  of  preservation  of  the  plume-like  masses  in 
the  spherulites  of  the  Cuillin  Hills  is  very  much  inferior  to  that 
of  the  spherulites  from  the  western  districts  of  the  United  States, 
microscopical  study  proves  that  they  are  identical  in  character  and 
origin  with  the  objects  so  well  described  by  Messrs.  Iddings  and 
Whitman  Cross.  I  have  no  doubt  that  those  observers  are 
correct  in  referring  all  the  fibres  of  these  arborescent  growths  to 
felspar ;  they  have  shown,  by  the  use  of  the  quartz-plate  a  teinte 
sensible ,  in  the  manner  suggested  by  M.  Michel-Levy,  that  the 
positive  or  negative  character  of  the  spherulitic  growth  may  be 
accounted  for  by  regarding  the  former  as  felspars  elongated  parallel 
to  the  vertical  axis  and  with  the  so-called  ‘  anomalous  ’  optical 
orientation,  and  the  latter  as  being  also  felspar-crystals,  but  elon¬ 
gated  parallel  to  the  clino-axis  and  with  the  normal  optical  orien¬ 
tation.  These  felspar-mi crolites  are  frequently  Manebach  twins, 
and  this  accounts  for  the  very  regular  forking  of  the  fibres  at 
definite  angles.  The  whole  character  of  the  spherulites  of  the 
Cuillin  Hills  is  in  complete  harmony  with  this  explanation. 
Although  quartz  does  not  enter  directly  into  the  composition  of 
these  spherulitic  growths,  as  was  formerly  supposed,  free  silica  takes 
a  small,  but  by  no  means  unimportant  part  in  the  building-up  of 
such  structures.  Mr.  Iddings  has  shown  that  the  fibres  of  felspar 
entangle  numerous,  small,  rosette-like  aggregates  of  tridymite, 
while  Mr.  Whitman  Cross  points  out  that  the  spaces  between  such 
fibres  may  be  filled  in  with  colloid  silica,  probably  hydrated.  Both 
tridymite  and  colloid  silica  have  of  course  long  since  disappeared  in 
the  altered  spherulites  of  the  Cuillin  Hills  ;  but  the  crystallized  quartz 
between  the  felspar-fibres  probably  represents  the  silica  which  has 
been  redissolved  and  has  crystallized  in  the  stable  form.  We  find 
an  interesting  proof  of  the  former  existence  of  the  tridymite 
rosettes  when  we  attempt  to  polish  the  sections  of  these  spherulites, 
the  polishing  powder  finding  its  way  into  and  rendering  very-  con¬ 
spicuous  the  cavities  formerly  occupied  by  tridymite. 
There  are  two  minerals  formed  in  these  spherulites  which  are  of 
especial  interest.  The  first  of  these  is  iron  pyrites.  In  my  paper 
of  1874  I  pointed  out  how  frequently  this  mineral  is  present  in  the 
Tertiary  igneous  masses  of  the  Western  Isles  of  Scotland.1  At  a 
later  date,  I  described  the  propylitic  modification  of  the  older  ande¬ 
sites  by  the  intrusion  into  them  of  the  granitic  rocks.2 
The  masses  of  fused  granite  enclosed  in  the  gabbro  of  the  Cuillin 
Hills  all  contain  sulphur ;  this  has  been  shown  by  a  series  of  careful 
tests  with  the  blowpipe  made  for  me  by  Mr.  Gilbert  Cullis,  one  of 
the  Demonstrators  of  the  Geological  Division  of  the  Boyal  College 
of  Science.  But  in  addition  to  the  finely-divided  iron  sulphide  we 
1  Quart.  Journ.  Geol.  Soc.  vol.  xxx.  (1874)  pp.  236-237. 
2  Ibid.  vol.  xlvi.  (1890)  pp.  341-384. 
