Yol.  49.] 
OF  M USCO VITE-BIOTITE  GNEISS. 
351 
other  rock  which  affords  clear  evidence  of  increasing  metamorphism 
to  the  north  is  the  old  basic  sill.  Year  the  Great  Fault,  where 
sharply  folded,  this  is  a  dark  green,  fine-grained,  schistose  rock, 
mainly  composed  of  chlorite,  epidote,  and  carbonates.  Half-a-mile 
north  of  the  Great  Fault  it  has  gradually  passed  into  a  distinctly 
crystalline  chlorite-schist.1 
YI.  Sedihentary  Origin  of  the  Meiahorphic  Rocks. 
The  sedimentary  origin  of  the  group  of  rocks  last  described  will 
probably  be  admitted  without  question,  but  in  the  case  of  the  more 
highly- altered  rocks  it  is  not  so  clear,  and  we  have  to  depend  rather 
on  the  broader  structural  features  due  to  original  stratification  and 
on  the  chemical  composition  of  the  rocks  for  information  as  to  their 
original  characters.  Thus  the  lowest  rocks,  the  quartzites  of  the 
Yorth  Esk  valley,  are  highly  siliceous.  Like  the  quartzites  of  Ben-y- 
Ghlo  (Yorth-eastern  Perthshire),  they  contain  a  certain  amount  of 
felspar,  and  differ  from  the  latter  chiefly  in  the  greater  reconstruction 
that  they  have  undergone.  These  highly  siliceous  rocks  form  nearly 
one-fifth  of  the  whole  area  described.  The  quartzite  is  succeeded  by 
the  limestone — a  perfectly  normal  sequence — while  the  character 
of,  and  the  minerals  contained  in,  this  calcareous  rock  are  such  as 
the  researches  of  Prof.  Brogger  and  others  would  lead  us  to  expect. 
Yext  to  the  limestone  in  the  Tarfside  area  is  a  bluish-grey  hornfels- 
like  rock  which  contains  sillimanite,  and  its  cross-fracture  is  sugges¬ 
tive  of  its  being  an  altered  shale  or  clay ;  but  it  has  not  yet  been 
analysed.  There  is,  however,  a  mass  of  siUimanite-bearing  rocks 
extending  along  the  southern  side  of  the  Yorth  Esk  for  a  consider¬ 
able  distance,  as  to  the  origin  of  which  it  is  impossible  to  speak 
with  certainty.  But  it  may  be  fairly  said  that  the  evidence  of  the 
area  as  a  whole  points  to  their  sedimentary  origin,  and  there  is 
nothing  to  suggest  that  they  have  been  formed  of  crushed  igneous 
material.  The  great  mass  of  pebbly  grits  that  can  be  traced  to 
fairly  coarse  gneisses  has  already  been  mentioned  :  it  may,  however, 
be  here  added  that  the  felspar,  both  in  the  grits  and  the  gneisses, 
is  almost  exclusively  oligoclase. 
There  are  only  two  other  bands  of  importance  in  the  highly 
altered  area.  One  of  these  forms  the  long  outcrop  of  staurolite- 
schist  already  referred  to.  This  rock  contains  no  less  than  36*4 
per  cent,  of  alumina;2  consequently  it  could  only  have  been  shale  or 
clay  originally.  The  other  is  the  green-mica  quartzite,  which  often 
consists  almost  entirely  of  silica.  Thus  it  will  he  seen  that  the 
greater  part  of  the  series  was  probably  of  sedimentary  origin. 
1  [It  has  been  suggested  that  the  alteration  of  these  jaspers  and  green  chlo- 
r-itic  phyllites  (they  are  not  truly  crystalline  schists)  is  no  greater  than  that 
known  to  be  produced  by  earth-movement  alone,  unaccompanied  by  igneous 
intrusions.  It  is  doubtful  whether  such  may  not  be  the  case,  but  the  extreme 
clearness  of  the  minerals  in  fresh  specimens  of  the  chloritic  rocks  suggests 
incipient  thermometamorphism,  as  well  as  mechanical  deformation. — May  29th, 
1893.] 
2  See  Table  of  Analyses,  p.  355. 
