346 
MR.  G.  BARROW  ON  AN  INTRUSION 
[Aug.  1893, 
Since  the  quarry  was  opened,  the  rock  has  decomposed  to  a  depth  of 
several  inches  to  a  soft,  brown,  ferruginous  mass ;  the  unweathered 
rock  is  a  bluish-grey  limestone.  A  specimen  of  this  (4869)  shows 
calcite,  green  hornblende,  malacolite,  zoisite,  spliene,  felspar  (in¬ 
cluding  microcline),  and  iron  ores.  The  microcline  is  very  remarkable, 
because  we  have  never  yet  found  it  in  any  of  the  metamorphosed 
rocks  of  the  Central  Highlands, except  where  it  was  part  of  an  original 
crystal  of  an  igneous  rock,  as  the  Ben  Yuroch  granite.  There  is 
no  white  mica  in  the  limestone,  and  further  there  is  very  little  in 
the  rocks  next  to  the  limestone,  though  they  contain  much  brown 
mica.  White  mica  increases  rapidly  in  amount  as  we  leave  the 
limestone  :  it  is,  indeed,  doubtful  whether  this  mineral  is  ever  absent 
from  the  altered  sediments  of  this  area,  except  close  to  the  calcareous 
rocks.  Microcline  continues  to  be  abundant  in  the  limestone  over 
all  the  Tarfside  area.  The  malacolite  has  no  idiomorphic  boundaries, 
but  it  occurs  in  much  larger  individual  crystals  than  the  other  con¬ 
stituents. 
At  the  base  of  this  little  calcareous  series  are  one  or  more  bands 
of  siliceous  rocks,  which  weather  like  very  massive  quartzites. 
They  are  exclusively  confined  to  the  north-eastern  side  of  the  North 
Esk,  where  they  cover  a  great  area  and  are  the  lowest  rocks  yet 
known  to  occur  in  the  Southern  Highlands.  Erom  a  nearly  pure 
quartzite  they  vary  to  a  felspathic  phase,  in  which  both  white  and 
brown  mica  are  present.  Such  a  rock  is  seen  under  the  microscope 
to  be  markedly  foliated.  The  felspar  occurs  in  little  patches  shaped 
rather  like  a  double  wedge :  these  are  joined  together  by  their 
taper  ends  and  so  give  a  well-marked  foliated  aspect  to  the  rock, 
quite  independently  of  the  micas.  It  can  only  be  fairly  called 
a  muscovite-biotite  gneiss,  but  its  extremely  high  percentage  of 
free  silica  puts  its  sedimentary  origin  beyond  doubt. 
(b)  The  Cyanite-zone. 
Though  highly  crystalline,  the  rocks  of  this  zone  are  not  so  coarse 
in  texture  as  those  of  the  sillimanite-zone  in  the  same  neighbour- 
hood.  This  is  shown  both  by  the  size  of  the  micas  and  also  by  that 
of  the  individual  grains  composing  the  felspar-segregations'.  To 
begin  with  the  rocks  in  which  the  characteristic  mineral  is  met 
with:  cyanite  is  shown  in  a  slide  made  from  the  schist  close  to 
the  old  limestone-quarry  in  Glen  Clova ;  it  is  not  visible  in  a  hand- 
specimen.  This  schist  is  fairly  soft  and  finely  puckered,  being 
practically  a  seri cite- schist,  and  is  dotted  over  with  specks  of  brown 
mica.  Under  the  microscope  it  is  seen  to  consist  of  quartz  and  two 
micas,  the  flakes  of  white  mica  frequently  meeting  so  as  to  form  a 
number  of  small  angles ;  and  in  these  angles  the  brown  micas,  seen 
on  the  surface  of  the  rock,  mostly  lie.  They  are  the  typical  red- 
brown  contact-mica,  and  are  full  of  intensely  pleochroic  spots. 
In  addition  to  these  micas,  a  few  crystals  of  garnet,  and  crystals 
(or  more  often  aggregates)  of  cyanite  occur.  Earther  up  the  glen 
larger  crystals  of  cyanite  are  seen  in  the  fallen  blocks  of  gneiss, 
