Yol.  49.] 
OF  MT7SCOVITE-BIOTITE  GXJSISS. 
355 
The  brown  micas  are  both  rich  in  ferrous  oxide  and  low  in 
magnesia.  They  belong  to  the  haughtonite  group.  The  white  mica 
of  the  altered  rocks  contains  appreciably  more  soda  than  that  of  the 
igneous  rock  ;  but  the  water  of  crystallization  in  the  igneous  mica 
is  more  than  twice  as  great  as  in  the  contact-mica.  This  is  true  of 
both  brown  and  white  mica,  but  it  is  more  noticeable  in  the  latter 
case. 
The  grey  slate  or  phyllite  shows  the  high  percentage  of  alkalis 
(6*41)  which  the  finer  sediments  of  the  Central  Highland  Series 
contain.  This  rarely  falls  below  5,  and  often  reaches  7  per  cent. 
The  staurolite-schist  affords  a  perfect  example  of  a  rock  that 
can  be  proved  to  be  a  sediment  by  analysis.  The  percentage  of 
alumina  shows  that  it  must  have  been  a  clay  or  a  shale.  Ho  known 
igneous  rock  could  have  even  approximately  such  a  composition. 
The  iron  is  about  the  same  as  that  given  in  Prof.  Henard’s  analyses 
of  the  Ardennes  phyllite,  and  was  originally  in  the  form  of  pyrites. 
A. 
B. 
C. 
D. 
E. 
F. 
Si02  . 
....  34-90 
43-08 
35-00 
45-80 
58-00 
39-70 
■A1203  . 
....  2327 
32-85 
25-06 
3T84 
20-16 
36-40 
FeO  . 
....  20-87 
2-76 
15-30 
Pe203  . 
....  2-56 
•73 
3-94 
5-86 
7-64 
9-60 
CaO  . 
....  1-20 
1-07 
1-50 
Trace 
•73 
1-20 
MgO  . 
....  432 
•33 
6-48 
115 
2-49 
3  20 
k2o  . 
....  6-94 
8-78 
9-31 
7-56 
3-91 
3-60 
Na20  . 
....  2-01 
1-00 
1-84 
3-19 
3-41 
2-58 
Loss  . 
...  3-60 
912 
1-72 
4-90 
3-03 
4-50 
Total  . 
...  99-67 
99-72 
100-15 
100-30 
99-37 
100-78 
EXPLANATION  OF  THE  PLATES. 
(Map)  Plate  XV. 
For  many  miles  to  the  S.W.  the  Central  Highland  rocks  have  been  found  to 
be  pierced  by  a  number  of  coarse  pegmatite-veins,  the  origin  of  which  was 
uncertain.  They  have  now  been  clearly  traced  to  the  pareut  mass,  a  muscovite- 
biotite  gneiss,  from  which  they  proceed.  The  Map  shows  the  only  known  out¬ 
crops  of  this  parent  mass  in  the  South-eastern  Highlands.  To  the  N.W.  the 
gneiss  is  let  down  by  an  enormous  fault,  but  in  all  other  directions  it  is  sinking 
very  slowly  beneath  the  crystalline  schists,  the  upper  limit ,  however,  being 
extremely  uneven. 
The  small  patches  in  the  sillimanite  are  upward  prolongations  of  the  type  of 
No.  2  or  the  centre  of  No.  3.  No  other  outcrop  of  No.  4,  the  southerly  termi¬ 
nation  of  the  intrusion,  is  known.  The  zones  of  staurolite-  and  cyanite-gneiss 
or  schist  really  represent  the  variation  in  height  above  the  upper  limit  of  the 
underlying  gneiss. 
The  great  fault,  shown  to  the  N.W.,  crushes  and  greatly  alters  these  ineta- 
morphic  rocks,  while  the  newer  granite  and  diorite  are  more  recent  than  this 
fault,  as  the  Map  clearly  shows  in  the  case  of  the  diorite.  Further,  these 
newer  intrusions  largely  destroy  the  older  crystallization,  as  may  be  seen  in 
G-len  Clova.  (Shown  in  the  scars  a  little  above  ‘  Gf  ’  of  ‘  G-len  ’  on  the  Map.) 
Specially  good  places  for  observing  the  minerals  sillimanite,  cyamte,  and 
staurolite  are  shown  by  the  letters  S,  C,  and  St.,  and  good  outcrops  of  lime¬ 
stone  by  the  letter  L. 
