Yol.  49.] 
OF  MUSCOYITE-BIOTITE  GNEISS. 
345 
micaceous  surfaces  almost  iu  the  form  of  little  cones  from  which  the 
apex  has  been  broken  with  a  jagged  fracture.  These  conical  projections 
vary  in  colour  from  green  to  yellow.  They  give  the  rocks  a  very 
jagged  or  roughened  surface,  which  can  be  easily  noticed  several 
yards  off.  A  cross-fracture  of  such  a  specimen  is  generally  compact, 
and,  if  fresh,  not  a  trace  is  seen  of  the  structure  brought  to  light  by 
weathering.  About  Tarfside,  a  rock  very  similar  to  the  last  in  weather¬ 
ing,  but  with  a  still  more  compact  fracture,  keeps  close  to  the  out¬ 
crop  of  the  limestone  which  was  at  one  time  extensively  quarried. 
There  is  every  reason  to  believe  that  a  large  proportion  of  the  rocks 
to  the  eastward  contain  sillimanite  in  the  ‘film’  phase,  associated 
with  frayed  brown  mica. 
In  the  neighbourhood  of  Millden,  on  the  North  Esk,  the  little 
projections  of  quartz  sillimanitise  are  frequently  met  with  on  the 
weathered  surfaces  of  the  rocks.  Fine  examples  of  rather  highly 
crystalline  sillimanite-bearing  rocks  rise  almost  like  dykes  out  of  the 
ground,  close  to  the  Meallie  Burn.  This  is  on  the  northern  side  of 
the  main  river.  On  the  southern  side,  somewhat  similar  rocks  are 
seen  above  the  large  farmhouse  of  Ealhastnie,  while  in  the  little 
Dalscampie  Burn,  farther  south-east,  the  least  crystalline  variety  of 
sillimanite-bearing  rocks  is  seen.  It  is  a  fine  crystalline  schist  in 
which  segregated  quartz  occurs  in  very  thin  films.  A  specimen 
(5107)  is  seen  to  be  composed  of  quartz,  chlorite,  and  wrhite 
mica,  with  larger  patches  of  quartz  fairly  free  from  the  other  com¬ 
ponents  of  the  rock.  Tying  parallel  with  the  schistosity  are  certain 
faintly-yellow  lenticular  patches,  closely  resembling  4  shimmer  ’- 
aggregates.  Their  centres  are  invariably  very  finely  fibrous,  and 
a  comparison  with  other  slides  leaves  little  doubt  that  these  patches 
are  faserkiesel  or  quartz  sillimanitise.  No  other  sillimanite-bearing 
rocks  are  known  to  occur  south-east  of  this  locality. 
There  is  in  this  zone  a  sill  of  basic  igneous  rock,  which  always 
occurs  close  to  the  limestone  (see  below,  p.  350).  In  the  most 
altered  areas,  i.  e.  the  most  gneissose,  this  sill  is  a  more  or  less 
coarse,  hornblendic  gneiss,  while  in  the  areas  where  the  felspar- 
segregations  are  of  a  less  coarse  type,  it  becomes  a  more  ordinary 
hornblende-schist. 
In  addition  to  the  rocks  described  above,  a  band  of  impure  crys¬ 
talline  limestone  is  frequently  met  with.  Its  colour  and  appearance 
vary  little  if  unweathered,  but  there  is  a  good  deal  of  difference  in 
the  decomposed  rock,  depending  on  the  presence  in  greater  or  less 
quantity  of  iron-bearing  silicates.  Along  the  Lee  Water  it  is  bright 
bluish -grey,  and  a  microscopic  section  shows  it  to  be  composed 
of  calcite,  green  hornblende,  zoisite,  felspar,  quartz,  sphene,  and 
pyrites.  Blue  tourmaline  is  rare. 
The  zoisite  is  well  marked  by  the  weak  indigo  tint  under  crossed 
nicols.  The  mica,  or  some  of  it,  does  not  look  like  the  ordinary  brown 
mica  of  the  district,  but  it  is  not  easy  to  isolate  a  sufficient  quantity 
for  analysis.  Much  of  the  sphene  is  in  roundish  grains.  A  very  inter¬ 
esting  type  of  the  limestone  is  seen  in  an  old  quarry  at  Stylemount, 
halfway  between  Loch  Lee  and  Tarfside,  going  by  the  old  road. 
Q.  J.  G.  S.  No.  195.  2  x 
