Yol.  49.] 
OF  THE  ISLAND  OF  CAPEAJA. 
141 
olivine  occurs  only  in  grains  and  fragments.  Here  and  there 
may  be  detected  small  quantities  of  a  doubly-refracting  colourless 
mineral  in  the  interstices  of  the  felspar- crystals,  which  gives 
between  crossed  nicols  the  characteristic  polarization-tints  of  nephe- 
line.  That  it  really  is  this  mineral  can  be  shown  micro-chemically 
by  the  use.  of  hydrochloric  acid  and  fuchsine.  The  nepheline  was 
undoubtedly  the  last  constituent  to  crystallize  out. 
As  regards  the  age  of  the  various  constituents,  I  should  arrange 
them  as  follows,  beginning  with  the  oldest (1)  Magnetite,  (2) 
Olivine,  (3)  Augite,  (4)  Mica,  (I)  Felspar,  (6)  Nepheline. 
6.  Chemical  Analyses  of  the  Rocks. 
In  choosing  specimens  for  analysis,  I  have  endeavoured  as  far  as 
possible  to  select  representatives  of  all  the  different  varieties  of 
andesite.1 
I. 
II. 
III. 
Si02 
56-82 
SiO„  60-39 
Si02 
60-63 
aia 
16-68 
AlA  16-96 
AlA 
Fe.,03 
16-96 
Fe.,03 
3-84 
Fe.,03  1  -50 
2-87 
FeO 
4-57 
FeO  3-42 
FeO 
2-31 
OaO 
6-36 
CaO  5-41 
CaO 
6-41 
MgO 
3-93 
MgO  3"81 
MgO 
3-27 
!Na90 
3-39 
Na.,0  3-37 
Na00 
3-58 
K.,0 
2-81 
ICO  2-01 
K,0 
2-44 
op 
traces 
TiO.,  0-21 
h;o 
1-98 
p2o5 
traces 
Cl0  traces 
H.,0 
1-48 
P.,0-  traces 
H  O  2  03 
100-45 
99-88 
Sp.  Gr.  at  22°  C.  2' 
Sp.Gr.at22°C.  2-657. 
8p.Gr.  at  22°  C.  2-580. 
No.  I.,  from  Monte  Scopa,  is  an  average  sample  of  the  darker,  less 
glassy  variety.  Felspar,  augite,  mica,  olivine,  and  magnetite  are 
porphyritically  developed  in  a  groundmass,  consisting  chiefly  of 
microliths  and  a  small  quantity  of  interstitial  glass. 
No.  II.,  from  Monte  Patello,  is  a  good  representative  of  the  more 
glassy  varieties.  The  felspar,  mica,  and  augite  are  very  fresh  and 
well  crystallized.  The  groundmass  is  rich  in  light-brown  isotropic 
glass,  and  microliths  of  felspar,  augite,  and  mica.  The  rock  is 
almost  free  from  magnetite. 
No.  III.,  from  Poppa  alia  Nave,  resembles  microscopically  a 
basalt.  The  porphyritic  constituents  are  comparatively  scarce : 
plagioclase,  olivine,  and  an  occasional  mica-flake  are  scattered  in  a 
black  and  very  compact  groundmass,  which,  under  the  microscope, 
is  resolved  into  augite-  and  felspar-microliths  and  a  little  inter¬ 
stitial  glass. 
1  All  the  following  analyses  were  made  for  me  by  Dr.  Rohrig,  in  his  private 
laboratory  at  Leipzig. 
