Yol.  49.] 
OF  MUSCOYITE-BIOTITE  GNEISS. 
337 
is  seen  to  be  dependent  on  nearness  to  or  remoteness  from  the 
igneous  masses ;  and  in  presence  of  sucb  evidence  it  seems  only 
reasonable  to  attribute  both  the  minerals  and  the  crystallization 
to  the  thermometamorphism  of  the  intrusion. 
IV.  Minerals  of  the  Met  amorphic  Rocks. 
In  the  following  petrological  descriptions,  the  minerals  that  are 
especially  suggestive  of  this  mode  of  origin  will  first  be  dealt  with, 
and  a  brief  account  will  then  be  given  of  the  rocks  in  which  they 
occur. 
(1)  Sillimanite  or  Fibrolite. — This  mineral  occurs  so  abundantly 
in  the  region  of  greatest  metamorphism  as  to  justify  the  application 
of  the  term  e  sillimanite-zone  ’  to  the  area  in  which  it  occurs.  Over 
at  least  50  square  miles  fully  one  half  of  the  rocks  contain  this  sili¬ 
cate  of  alumina,  and  usually  in  considerable  quantity.  Its  distri¬ 
bution  in  the  north-west  is  probably  limited  by  a  huge  fault  shown 
in  the  Map  (PI.  XV.),  but  to  the  south-east  sillimanite  does  not  occur 
far  from  the  acid  edge  (pegmatite)  of  the  great  intrusion.  As  the 
mineral  has  hitherto  been  so  little  known  to  most  British  geologists, 
it  may  not  be  out  of  place  to  give  a  brief  summary  of  the  previously 
published  accounts  of  it.  Of  the  earlier  writers,1  Dr.  E.  Kalkowsky, 
in  a  paper  on  the  ‘  Gneissformation  des  Eulengebirges,’  Leipzig, 
1873,  notes  the  presence  of  sillimanite-needles  in  quartz  ( faser - 
Ixiesel).  Several  communications  by  different  authors  appeared  soon 
after,  but  it  seems  that  Michel-Levy  was  the  first  to  recognize  its 
mode  of  production.  In  a  paper  of  two  pages  in  the  Bull.  Soc. 
Frang.  Mineral.  (1880)  he  gives  an  account  of  its  optical  properties 
and  chemical  composition.  Its  origin  is  then  traced  to  ‘  contact  *- 
action;  and  he  concludes  by  stating  that  Sainte-Claire  Deville  had 
succeeded  in  producing  it  artificially  by  heating  to  redness  a 
mixture  of  aluminium  fluoride  and  silica.  The  contact-origin  of 
this  mineral  has  been  further  proved  by  Dr.  Barrois's  papers  on 
the  Gueinene  and  Rostrenen  Granites,  published  in  the  Annales 
Soc.  Geol.  clu  Xord.2  In  these  cases  the  sillimanite  occurs  in 
sandstones  of  Lower  Silurian  age.  I11  the  Hautes  Pyrenees  Lacroix 
found  sillimanite  in  the  andalusite  of  the  maclites  (massive  anda- 
lusite-rocks),  and  he  also  ascribed  it  to  contact- action.3  In  this 
country  Prof.  Heddle  recorded  sillimanite  from  Pressendye  Hill  in 
Aberdeenshire,  and  also  from  Glen  Clova.  Soon  afterwards  Prof. 
Bonney  published  an  account  of  its  occurrence  in  the  Rock  of  the 
Black  Dog,  again  in  Aberdeenshire  ;  but  neither  of  these  writers 
made  any  suggestion  as  to  its  mode  of  origin.  In  1890  Miss  Gardiner 
communicated  to  this  Society  the  results  of  her  work  on  the  contact- 
1  A  list  of  the  German  memoirs  is  given  in  Rosenbusch’s  ‘  Mikroskopische 
Pbysiographie  ’  (art.  Sillimanite,  3rd  ed.  1892,  vol.  i.  p.  440). 
2  1884,  vol.  xi.  pp.  184  et  seqq.,  and  vol.  xii.  pp.  1  et  seqq. 
3  ‘  Roches  metamorphiques  et  eruptives  de  l’Ariege,’  Bull.  Serv.  .Carte  Geol. 
France,  No.  11,  vol.  ii.  (1890) ;  see  also  ‘Note  sur  une  association  de  sillimanite 
et  d'andaiousite,’  Bull.  Soc.  Frang.  Mineral,  vol.  xi.  (1888)  p.  150. 
