Yol.  49.] 
OF  THE  ISLAXD  OF  CXPEAJA. 
137 
general  habit  is  prismatic  along  II.  P.  Albite-twinning  is  universal, 
often  in  conjunction  with  the  perieline,  and  less  frequently  with 
the  Carlshad-twinning.  Except  in  the  case  of  some  of  the  dykes, 
the  felspars  do  not  contain  very  many  inclusions  :  often,  indeed, 
they  are  quite  free  from  them.  Glass  is  of  most  frequent  occur¬ 
rence,  either  with  the  contours  of  its  host — and  then  generally  with 
the  immovable  bubble — and  arranged  parallel  to  the  boundaries  of 
the.  crystal,  or  in  irregular  patches  scattered  at  random.  Hore 
seldom,  and  then  often  accompanied  by  evidences  of  magmatic  re¬ 
sorption,  is  the  felspar  completely  filled  with  what  looks  like  fine 
dust  under  low  powers  ;  or  else  a  broad  band  of  the  same  envelopes 
a  clear  felspar-kernel.  In  either  case  there  is  an  outermost  zone  of 
limpid  felspar-material,  showing  evidences  of  an  attempt  on  the  part 
of  the  crystal  to  regain  sharp  outlines.  In  places  where  the  £  dust  ’ 
is  coarser,  it  can  sometimes  be  resolved  into  long,  narrow,  glass  in¬ 
clusions,  which  show  a  tendency  to  arrange  themselves  with  their 
axes  parallel  to  the  edge  II,  P.  Xext  to  glass,  mica  is  the  commonest 
inclusion,  and  is  often  macroscopic-ally  recognizable.  Augite  and 
magnetite  occur  much  less  frequently. 
(ii)  Mica. — This  mineral  occurs  in  hexagonal  plates,  with  well- 
defined  crystalline  boundaries.  The  colour  ranges  from  light  straw 
yellow  to  dark  red  :  the  former  generally  in  the  dykes,  the  latter 
onlv  in  those  andesites  whose  groundmass  is  turning  or  has  alreadv 
turned  red.  The  pleochroism  is  strong,  and  in  the  fighter  varieties 
the  polarization-colours  are  brilliant.  In  directions  at  right  angles 
to  the  cleavage  the  twinning  is  often  very  well  marked,  between 
crossed  nicols.  by  the  difference  in  the  angle  of  extinction  in  neigh¬ 
bouring  lamellae.  In  manv  cases  basal  sections  are  traversed  by 
three  systems  of  cracks  intersecting  each  other  at  an  angle  of  60'  — 
as  pointed  out  by  Hussak  in  the  Gleichenberg  trachyte.  The 
mica  carries  magnetite,  apatite,  augite,  and  felspar  as  inclusions. 
The  latter  is  by  no  means  uncommon,  and  is  interesting  as  showing 
the  contemporaneous  crystallization  of  felspar  and  mica.  The  same 
mav  be  said  of  the  augite,  which  both  carries  mica  as  an  inclusion 
and  itself  occurs  in  mica.  In  many  cases,  as  usual,  the  mica  is 
entirely  or  in  part  replaced  by  magnetite  and  augite,  or  has  only  an 
exterior  ring  of  them. 
yiii)  Augite. — The 'crystal-faces  have  already  been  mentioned  as 
maeroscopically  visible.  Twins  after  the  orthopinacoid  are  very 
common  ;  those  after  a  hemi pyramid  much  less  so.  Sections  are 
either  colourless  or  of  the  palest  green,  without  a  trace  of  pleo¬ 
chroism.  Inclusions  are  not  very  abundant,  as  a  rule  ;  where  they 
occur,  they  are  either  glass,  mica,  augite-mic-rofiths,  or  magnetite. 
A  faint  opacite-ring  was  observed  in  one  or  two  cases.  As  already 
pointed  out,  mica,  felspar,  and  augite,  each  occurs  as  the  including 
or  included  mineral,  proving  that  all  three  must  have  crystallized 
from  the  molten  magma  at  about  the  same  time.  Augite  also  occurs 
as  an  inclusion  in  felspar,  but  I  was  unable  to  find  a  single  instance 
of  a  felspar  in  the  augite. 
(iv)  Hypeesthene. — This  mineral  has  a  prismatic  habit,  with  the 
