142 
ME.  H.  EMMONS  ON  THE  PETEOGEAPHY  [May  1 893, 
»p. 
IY. 
V. 
VI. 
Si02 
62-27 
Si02 
59-06 
Si0o 
59-29 
A1A 
16-92 
A1203 
16-40 
A1A 
15-27 
Fe[Os 
2-40 
Fe203 
2-88 
Fe203 
5-21 
FeO 
259 
FeO 
4-18 
FeO 
2-08 
CaO 
4-78 
CaO 
4-32 
CaO 
615 
MgO 
2-87 
MgO 
2-63 
MgO 
4-42 
Na„0 
4-72 
Na.,0 
5-29 
Na.,0 
3-31 
k,o 
1-47 
k2o 
1-49 
K,0 
2-61 
TiO„ 
0-16 
Ti02 
TfO, 
Cl2  “ 
007 
Cl2 
traces 
CL, 
0-036 
traces 
PA 
traces 
PA 
0-18 
h;o 
1-22 
h;o 
2-06 
H„0 
1-46 
99-47 
98-31 
100-016 
Gr.  at  22°  C.  2-584. 
Sp.  Gr.  at  22°  C.  2-606.  Sp.  Gr.  at  22°  C.  2-614. 
Nos.  IV.  and  Y.  are  analyses  of  grey  andesite-dykes — No.  IY. 
from  Punta  della  Civitata  on  the  eastern,  No.  Y.  from  Punta  della 
Manza  on  the  western  coast.  In  appearance,  the  rocks  are  very 
similar ;  under  the  microscope,  however,  an  interesting  difference 
is  observable.  The  chief  porphyritic  constituents  in  both  are  fel¬ 
spar,  mica,  and  augite  ;  both  also  contain  hornblende,  but  under 
different  conditions.  In  No.  IY.  it  is  one  of  the  first  constituents 
that  has  crystallized  out,  but  in  small  quantities  ;  in  No.  Y.  it  is 
one  of  the  last,  and  is  in  fact  to  be  found  only  in  the  groundmass, 
but  there  in  considerable  quantities. 
The  groundmass  of  both  contains  much  glass,1  felspar,  augite,  and 
very  little  magnetite ;  while  in  No.  IY.  some  mica  is  to  be  found. 
No.  YI.  is  an  andesite  in  which  the  groundmass  has  turned  red, 
but  which  in  other  respects  is  still  fresh.  Felspar,  hornblende, 
mica,  augite,  and  magnetite  are  richly  distributed  throughout  the 
rock.  The  hornblende  and  mica  are  very  red  ;  the  porphyritic 
augite  is  still  fresh  or  has  only  a  thin  red  crust ;  the  augite  of  the 
groundmass,  however,  has  turned  completely  red.  The  groundmass 
shows  everywhere  a  deposition  of  hydrated  ferric  oxide. 
Three  of  the  rocks  whose  bulk-analyses  have  just  been  given, 
namely,  Nos.  I.,  II.,  and  Y.,  after  having  been  reduced  to  coarse 
powder,  were  fractionated  by  means  of  Thoulet’s  solution,  in  order 
to  separate  the  groundmass.  The  latter,  after  careful  purification, 
was  subjected  to  analysis,  with  the  following  results  : — 
I  A. 
II  A. 
<1 
> 
<0 
d 
SiO, . 
ai.A  . 
.  64-54 
.  17-36 
68-60 
17-27 
68-11 
15-56 
O 
l-H  a 
~  02 
r_^  Ol 
Fe0Oo  . 
. .  2-33 
2-09 
0-96 
O' 
0  d 
FeO . 
.  2-50 
0-585 
2-12 
P4 
03  d 
CaO . 
.  3-97 
1-72 
2-91 
%  © 
f_,  © 
MgO . 
.  1-56 
0-396 
1-10 
© 
Na20  . 
.  3-58 
3-30 
3-43 
5  » — ! 
K20 . 
.  2-54 
4-61 
2-61 
O 
h20 . 
.  1-50 
2-08 
2-82 
O 
99-88 
100-651 
99-62 
1  ISo.  Y.  was  taken  from  the  salband  of  the  dyke,  where  the  proportion  of 
groundmass  was  greater,  an  analysis  of  the  latter  being  thought  desirable. 
