136 
ME.  H.  EMMONS  ON  THE  PETROGEAPHY  [May  1893, 
hand-lens  these  may  be  occasionally  resolved  into  octagonal  prisms, 
whose  cleavage-faces  often  have  a  metallic  iridescence.  The  mineral 
proves  under  the  microscope  to  be  olivine.  Decomposition  turns  the 
rock  to  a  dark-green  mass,  and  the  olivine  to  rusty-brown  spots. 
The  second  variety,  of  which  all  the  thin  lava-streams  of  the 
southern  end  of  the  island  consist,  is  very  hard  and  compact,  and  of 
a  dark  grey-blue  colour,  the  olivine-mottling  being  as  well  marked 
as  in  the  first  variety.  It,  like  the  andesite,  shows  the  first  effects 
of  weathering  by  reddish  streaks  and  patches,  which  increase  until 
the  whole  rock  becomes  red.  This  variety,  in  all  but  the  thickest 
of  the  lava-streams,  has  an  amygdaloidal  structure  :  the  cavities 
have  been  either  filled  or  encrusted  with  carbonate  of  lime. 
4.  Microscopic  Characters  of  the  Andesites. 
As  already  noted,  these  rocks  may  be  divided,  microscopically 
as  well  as  macroscopically,  under  the  heading  of  andesites  and 
anamesites.  The  andesites,  however,  taking  both  the  outflows  and 
the  dykes  together,  may  be  subdivided  according  to  the  quantity 
and  variety  of  the  minerals  present. 
Mica  is  always  present,  and  generally  in  a  very  considerable 
quantity ;  so  also  a  monoclinic  pyroxene,  though  less  abundantly. 
In  addition  to  these,  hornblende,  olivine,  and  hypersthene  may  be 
present.  The  rocks,  then,  are  primarily  augite-mica- andesites,  and 
may  be  classified  as  follows  : — (1)  Augite-mica- andesites  ;  (2)  Horn¬ 
blende-bearing  augite-mica-andesites  ;  (3)  Olivine-bearing .  augite- 
mica- andesites  ;  (4)  Hypersthene-bearing  augite-mica-andesites. 
In  one  instance  only,  i.  e.  in  the  rock  from  the  top  of  Punta  del 
Trattojo,  is  the  hypersthene  developed,  at  the  expense  of  the  augite 
and  mica,  to  such  an  extent  as  to  suggest  the  classification  of 
that  rock  as  a  hypersthene-andesite. 
At  the  same  time,  there  is  no  intention  on  the  writer’s  part  to  lay 
very  much  stress  upon  the  above  subdivisions.  The  same  dyke,  which 
in  one  place  is  an  augite-mica-andesite,  becomes  a  few  yards  off  an 
olivine  or  hornblende-augite-mica-andesite.  The  presence  of  a  small 
quantity  of  olivine  or  hornblende  does  not  alter  the  general  character 
of  the  rock,  and  is  doubtless  attributable  to  slight  local  variations 
in  the  basicity  of  the  magma  during  consolidation. 
I  now  describe  the  chief  characteristics  of  the  minerals  occurring 
in  the  andesites. 
(i)  Felspar. — The  felspars  are,  without  exception,  triclinic,  and 
belong  in  the  majority  of  cases  to  the  more  basic  varieties.  Occa¬ 
sionally,  however,  crystals  are  found  with  a  very  small  extinction  on 
P  (001).  Zonal  structure  is  frequent,  the  number  of  zones  being 
ften  very  great ;  but  the  exterior  zones  do  not  seem  to  indicate  that 
the  felspar  as  a  whole  has  become  more  acid.  Crystalline  contours 
are  generally  well-preserved  and,  except  in  some  of  the  dykes,  show 
but  slight  traces  of  magmatic  corrosion,  still  less  of  fracture.  The 
faces  M,  P,  T,  1,  x,  y  are  those  most  frequently  recognizable.  The 
