Vol.  49. j 
OF  IHE  ISLAXD  OF  CAPBAJA. 
143 
The  amount  of  material  at  present  published  does  not  warrant 
the  drawing  of  positive  conclusions  concerning  the  relations  between 
rock  and  its  groundmass.  But  it  is  interesting  to  compare  the 
above  results  with  those  published  by  Lagorio  1  for  rocks  of  similar 
ac-iditv.  and  to  see  how  nearlv  they  agree  with  his  deductions. 
Here,  as  with  him.  there  is  a  marked  increase  in  the  percentage 
of  silica  in  the  groundmass.  and  a  decrease  of  iron,  calcium,  and 
magnesium.  In  two  of  the  analyses  (II  a  and  T a)  there  is  an  in¬ 
crease  of  potassium,  as  was  to  be  expected :  the  third  (I  a),  how¬ 
ever,  shows  a  slight  decrease,  for  which  I  am  unable  to  account, 
unless  it  be  due  to  the  fact  that  the  groundmass  of  this  roc-k  con¬ 
tains  much  less  glass — in  other  words,  has  cooled  more  slowly — 
than  the  other  :  or  that  the  hand-specimen  under  analysis  came 
from  a  part  of  the  outflow  which,  for  some  local  reason,  had  been 
rendered  poorer  in  potassium  than  the  main  mass. 
During  the  fractionation  of  Xos.  II.  and  V.,  those  portions  that 
sank  between  the  specific  gravities  of  2- 6  and  2*75  were  set  aside 
and  afterwards  refractionated  to  obtain  the  felspar.  The  greater 
portion  remained  suspended  in  a  solution  of  sp.  gr.  2-67.  This, 
after  careful  purification,  was  subjected  to  analysis  with  the  results 
tabulated  below.  They  approximate  to  a  labradorite  of  the  com- 
position  Ab.An., 
the  theoretical 
composition  of 
which  is  given  for 
comparison.  Lime  is  the  only  constituent  that 
shows  much  devia- 
tion  from  the  required  percentage. 
Theoretical 
Hb. 
Yb. 
Ab.  An. 
SiO, . . . . 
.  55-92 
56-25 
55-92 
Al.,0.3  . 
.  28-58 
28-56 
28-10 
Fe,(X  , 
.  1-00 
0-77 
CaO.... 
.  665 
6'54 
10-06  - 
MgO..., 
.  0-55 
1-06 
Xa,0  . 
.  5-66 
542 
5-92 
HO.... 
.  0-66 
0-61 
h;o... 
.  0-69 
0-33 
99-71 
99-54 
100-00 
The  following  three  analyses 
southern  end  of  the  island  : — 
are  from  specimens  collected  at  the 
YU. 
Yin. 
IX. 
Si(b . . . 
.  53-86 
50-34 
44-40 
ALA  . 
.  16-44 
16-72 
12-98 
FeA  ■ 
.  8-02 
15-12 
7-36 
FeO..., 
.  1-96 
CaO.... 
.  8-53 
8-00 
MgO  . 
.  5-14 
3-96 
11-60 
Xa,0 
.  4-52 
3-78 
kA... 
.  0-067 
1-83 
Cl,  ... 
0-035 
p.,03... 
.  0-15 
h;o... 
.  1-27 
0-72 
2404 
100-292 
100-47 
100-38 
Sp.  Or.  at  22°  C 
L  2-894. 
- 
1  ‘Leber  die  Xatur  der  Grlasbasis,’  Tscherm.  ILin.  u.  Petr.  Hitth.  vol.  viii. 
1887,  p.  467. 
