354 
MR.  G.  BARROW  ON  AN  INTRUSION 
[Aug.  1893, 
Summary  of  Results. 
1.  In  the  area  drained  by  the  higher  branches  of  the  Forfarshire 
North  Esk  and  South  Esk,  there  are  a  number  of  outcrops  of  an 
igneous  rock  varying  both  in  structure  and  composition. 
2.  The  variations  in  structure  are  due  to  the  presence  or  absence 
of  foliation  ;  while  those  of  composition  are  due  to  changes  in  the 
relative  proportions  of  the  potash-bearing  constituents  (microcline 
and  white  mica)  and  those  constituents  in  which  the  characteristic 
bases  are  soda  and  iron  (oligoclase  and  biotite). 
3.  As  the  various  intrusive  masses  are  followed  towards  the 
south-east  the  amount  of  potash-bearing  constituents  increases,  and 
the  soda-felspar  (oligoclase)  diminishes. 
4.  The  masses  are  frequently  fringed  with  pegmatite  (in  which 
the  potash-bearing  constituents  attain  a  maximum),  especially  on 
their  southern  and  eastern  borders. 
5.  The  fact  that  these  pegmatites  occur  over  a  very  large  area  in 
the  South-eastern  Highlands  suggests  that  a  great  portion  of  the 
district  must  be  underlain  by  the  intrusion  of  which  they  form  a  part. 
6.  The  rocks  that  surround  the  areas  in  which  the  intrusive 
masses  are  exposed  at  the  surface,  are  in  a  coarsely-crystalline 
condition. 
7.  This  coarsely-crystalline  character  decreases  as  the  rocks  are 
followed  towards  the  south-east. 
8.  Three  zones  characterized  by  the  abundance  of  the  three 
minerals,  sillimanite,  cyanite,  and  staurolite,  may  be  roughly  mapped 
out  in  the  North-west  Forfarshire  area. 
9.  The  sedimentary  character  of  the  metamorphic  rocks  as  a 
whole  is  established  by  their  chemical  composition.  Limestones, 
shales,  quartzites,  and  coarse  grits  may  all  be  recognized  in  the 
metamorphic  zones. 
10.  A  special  feature  in  the  altered  rocks  is  the  abundance  of 
oligoclase.  This  is  explained  by  the  frequent  occurrence  of  this 
mineral  in  the  grits  of  the  Highland  border,  and  by  the  presence  of 
a  high  percentage  of  alkalis  in  the  phy llites.  The  potash  which  was 
present  in  the  original  rocks  is  entirely  absorbed  in  the  formation 
of  the  micas. 
IX.  Analyses  of  the  Hocks. 
The  appended  table  of  analyses1  shows  the  composition  of 
A.  The  brown  mica  of  the  igneous  rock. 
H.  The  white  mica  of  the  igneous  rock. 
C.  The  brown  mica  of  contact-metamorphism. 
H.  The  white  mica  of  contact-metamorphism. 
E.  The  grey  slate  or  phyllite,  north  of  the  Great  Highland 
Fault. 
F.  The  staurolite-schist  north  of  Bulg. 
1  Analyses  E  and  F  were  made  by  Mr.  A.  Dick,  Jun.,  who  moreover  kindly 
helped  me  with  the  others. 
