420  DR.  C.  CALLAWAY  ON  THE  ORIGIN  OF  THE  [Aug.  1893, 
analyses,  winch  are  from  less  chloritic  specimens,  yield  respectively 
1*29  and  0*35  per  cent,  of  magnesia. 
While  lime  and  magnesia  are  gradually  eliminated  in  the 
shearing-process,  the  alumina  and  alkalies  are  tolerably  persistent. 
Prom  this  it  follows  that,  in  the  extreme  of  metamorphism,  there 
must  be  much  free  silica,  and  a  part  of  the  remainder  will  be  in 
combination  with  a  highly  aluminous  base.  Hence  the  abundance 
of  white  mica. 
Another  series  of  analyses  by  Mr.  Player  furnishes  similar  results 
as  regards  the  loss  of  magnesia.  They  illustrate  the  chemical 
changes  undergone  in  the  conversion  of  the  medium-black  diorite 
into  a  gneissoid  quartzite. 
V. 
VI. 
VII. 
VIII. 
Silica  . 
.  47T 
47-2 
66 ‘0 
8P9 
Alumina  . 
.  18V 
17-8 
16-6 
9-2 
Titanic  oxide . 
•4 
•4 
Trace. 
Ferric  oxide  . 
.  30 
6-7 
16 
•6 
Ferrous  oxide  . 
.  8-5 
56 
4-2 
16 
Manganic  oxide . 
•1 
... 
•  . . 
.  .  . 
Lime  . 
.  6-6 
4-8 
•5 
•9 
Magnesia  . 
.  7-3 
6-3 
1-9 
Soda  . 
.  2-4 
2-2 
P5 
1-6 
Potash  . 
.  2-8 
39 
5-3 
21 
Loss  . 
.  3-6 
4V 
2-2  . 
IT 
99-9 
99-0 
99-8 
99-5 
V.  Medium-black  diorite,  Swinyard’s  Hill. 
VI.  ‘  Basic  rock  ’  of  Mr.  Player.  It  is  the  same  variety  of  diorite,  but  it 
has  undergone  some  change,  a  parallel  structure  having  been  produced, 
and  a  little  biotite  has  been  generated.  Malvern. 
VII.  Biotite-gneiss  formed  out  of  the  same  diorite  at  contact  with  granite- 
veins.  Traced  gradatim  into  V. 
VIII.  Gneissoid  quartzite ;  a  more  advanced  stage  of  VII.  Swinyard’s  Hill. 
It  is  probable  that  some,  at  least,  of  this  magnesia  has  been 
removed  as  carbonate  ;  hut  I  have  not  been  able  to  find  dolomite 
or  magnesite  in  the  rock,  and  I  desire  to  confine  myself  to  facts 
actually  observed  or  proved  by  experiment. 
The  Fall  in  the  Percentage  of  Alumina. — The  final  stage  in  the 
conversion  of  diorite  into  an  acidic  gneissic  rock  is  accompanied  by 
a  great  diminution  in  the  alumina.  In  the  second  series  of  analyses 
(V.-VIII.)  it  will  be  seen  that  this  base  falls  to  about  one-half. 
As  it  may  be  contended  that  this  reduction  may  be  due  to  an  original 
difference  of  magmas,  a  series  of  analyses  has  been  very  kindly  made 
for  me  by  Mr.  T.  H.  Waller,  B.Sc.,  from  samples  taken  from  the  same 
block,  which  reveals  under  the  microscope  a  perfect  gradation  between 
the  biotite-gneiss  and  the  gneissoid  quartzite.  One  of  the  slides 
shows  the  sample  X.  (which  is  intermediate  between  biotite-gneiss  and 
gneissoid  quartzite,  and  displays  very  clear  kersantite-structure) 
passing  into  the  gneissoid  quartzite  (XI.),  which  differs  chiefly  in  being 
much  more  quartzose,  and  still  retains  traces  of  kersantite-structure. 
