EXPLANATION  OF  PLATE  XIX. 
Fig.  1.  Illustrates  the  ordinary  characters  of  the  glassy  rock  formic c  the 
centre  of  the  Cir  Mhor  dyke.  The  groundmass  shows  numerous  fern¬ 
like  aggregates  of  kornblende-microlites,  each  surrounded  by  its  ‘  court 
of  crystallization.’  In  the  lower  part  of  the  figure  are  seen  two  crystals 
of  felspar, which  in  polarized  light  exhibit  the  characters  of  anorthoclase. 
On  the  right-hand  side  is  an  ordinary  spherulite,  composed  of  bifur¬ 
cating  felspar-microlites,  surrounding  a  nucleus  formed  of  plates  of 
tridyraite  ;  this  spherulite  exhibits  positive  double  refraction.  Near 
the  centre  of  the  figure  are  two  grains  of  quartz,  enclosed  in  a  double 
envelope  of  perfectly  clear  hyalite,  with  negative  double  refraction. 
The  clear  substance  of  the  hyalite  is  penetrated  by  numerous  radiating 
microlites  of  hornblende.  Two  other  similar  globules  of  hyalite  are 
seen  in  the  figure: — one  on  the  left,  enveloping  a  minute  "group  of 
tridymite-crystals,  and  another,  in  the  lower  part,  attached  to  an  angle 
of  the  large  felspar-crystal. 
Fig.  1  a  shows  the  group  of  quartz-crystals,  with  their  globular  envelopes  of 
B  hyalite,  as  seen  with  a  higher  power,  between  crossed  nicols. 
Fig.  lb  is  a  drawing  of  portion  of  a  double  envelope  of  hyalite,  as  seen  with  a  high 
power,  in  a  Yery  thin  section  and  with  suitable  illumination.  Between 
the  two  layers  of  hyalite  are  seen  numerous  plates  of  tridymite,  and  the 
clear  substance  of  the  former  is  seen  to  be  traversed  by  many  very  fine 
hornblende-microlites. 
Fig.  ...  Augite-andesite  of  the  sides  of  the  Cir  Alhor  dyke.  The  rock  consists 
of  numerous  felspar-microlites,  crystals  of  brown  augite,  and  grains  of 
magnetite,  with  glass  between  them,  and  a  few  large  and  often  much 
corroded  porphyritic  crystals  of  nlagioclase.  In  the  upper  part  of  the 
figure  is  an  irregular  patch  of  tachy  lytic  glass,  filled  with  magnetite  dust  ; 
glass  also  occupies  an  amygdaloidal  cavity  near,  but  here  it  has  under¬ 
gone  much  alteration  ;  among  the  secondary  minerals  formed  from  the 
decomposition  of  the  glass  in  this  cavity,  chlorites  are  conspicuous. 
Discussion. 
Prof.  Bonnet  said  that  his  own  observations  in  Arran  several 
years  ago  enabled  him  to  confirm  Prof.  Judd’s  description.  He 
had  seen  basalt  (or  augite-andesite)  following  qnartz-felsite,  and 
pitchstone  following  quartz-felsite,  and  so  on  :  evidently  the  one 
rock  had  been  solid  when  the  other  came  ;  so  the  magma  must 
have  been  differentiated  underground.  He  quite  agreed  with 
Prof.  Judd  that  sometimes  differentiation  took  place  in  the  in¬ 
trusive  mass  itself,  but  that  separation  also  took  place  below,  and 
masses  of  different  composition  were  ejected  at  different  times. 
Mr.  W.  W.  Watts,  referring  to  the  occurrence  of  many  complex 
dykes  in  another  region,  the  coast  of  the  Mourne  Mountains, 
thought  that  the  exceptional  nature  of  the  phenomena  might  be 
explained  by  the  fact  that,  although  lines  of  weakness  in  and 
along  dykes  were  frequent,  the  necessary  materials  for  filling  them 
were  not  usually  so  near  at  hand  as  in  the  Eocene  period.  He 
enumerated  a  series  of  characters  which  ought  to  be  more  common 
in  igneous  rocks  if  segregation  during  crystallization  were  of 
frequent  occurrence  in  their  origin. 
Mr.  Hulke  and  Mr.  Teall  also  spoke}  and  the  Author  briefly 
replied. 
