Yol.  49.]  THE  LLETN,  AND  ASSOCIATED  VOLCANIC  ROCKS.  159 
lent  me  for  comparison  by  Prof.  Bonne}-.1  The  minerals  within 
amygdules  and  veins  include  epidote,  quartz,  a  sheaf-like  zeolite, 
calcite  and  other  carbonates,  and  iron  oxides.  Doubtless  in  many 
cases  these  substances  have  been  transferred  from  the  rock,  as  in 
a  dull-greenish  diabase  (north  of  Porth  Oer)  which  has  the 
structure  of  the  basalt  but  not  its  colour,  and  contains  small  red 
amygdules,  apparently  filled  by  material  drawn  from  the  surround¬ 
ing  mass.2  Some  of  the  secondary  deposits  have  been  modified  sub¬ 
sequently,  and  one  veined  specimen  was  described  by  Prof.  Bonney, 
as  illustrating  the  effects  of  pressure  upon  calcite.3 
A  carbonate  of  Time,  magnesia,  or  iron,  is  one  of  the  commonest 
secondary  products.  Prom  microscopic  study  much  of  this  appears 
to  be  dolomite  or  chalybite,  although  calcite  is  also  present.4 5  In 
many  diabase-tuffs,  it  is  scattered  through  the  slide  in  small,  clearer, 
rhomboidal  spaces,  often  including  reddish  haematite.  In  the 
microcrystalliue  basalts,  the  secondary  development  is  discovered 
only  on  application  of  polarized  light,  when  a  number  of  bright- 
coloured  rhomboids  suddenly  start  into  view.  These  consist  of  the 
carbonate  replacing  lath-shaped  felspars,  but  interrupted  by  the 
iron  oxide.  The  deposit  seems  at  places  to  creep  up  the  felspar- 
microliths,  so  as  to  project  beyond — and  cause  a  slight  irregularity 
in — the  boundary-line  of  the  rhomboid  (PI.  I.  fig.  1).  Similar 
crystals  are  deposited  in  the  Dinas-fach  breccia  within  the  felspathic 
rays  ;  while  elsewhere  the  wavy  felspar- microliths  of  a  small  radial 
group  have  been  replaced.'  These  pseudomorphosed  spherulites,  in 
polarized  light,  contrast  with  the  surrounding  duller  mass  by  their 
bright  pink  and  green  tints. 
Much  of  the  change  described  is  evidently  due  to  infiltration, 
since  rhombohedra  of  a  carbonate,  sparsely  scattered  in  the  mass, 
are  thickly  clustered  along  fissures  or  veins.  Sometimes  the  rock 
appears  to  have  been  saturated  throughout,  the  interchange  of  sub¬ 
stance  causing  in  both  masses  alterations  of  colour  which  can  easily 
be  recognized.  The  limestone  generally  assumes  a  pale  peach  tint, 
and  fragments  of  basalt  cemented  by  calcareous  veins  exhibit  an 
outer  zone  of  thin  alternating  laminae,  then  a  band  of  deep  red, 
while  the  interior  retains  a  normal  duller  tint. 
The  limestones  which  belong,  as  I  believe,  to  the  original  series 
of  rocks,  would  probably  be  a  sufficient  source  for  the  calcareous 
deposits,  without  imagining  the  extension  of  Carboniferous  or  other 
masses  of  more  uncertain  age,  and  iron  oxide  must  have  been 
present  in  large  proportion  within  the  basalts. 
1  Compare  Fouque  &  Michel-Levy,  ‘  Mineralogie  Micrographique,’  1879, 
pis.  xxviii.  and  xxix. ;  U.  S.  Geol.  Explor.  of  40th  Par.  vol.  vi.  ‘  Micr.  Petr.,’ 
1876,  F.  Zirkel,  pi.  iv.  fig.  2,  &  pi.  v.  fig.  2. 
2  Eadiate  clusters  of  lath-shaped  crystals  project  into  the  vesicles  along  their 
ed^e,  the  remainder  of  the  space  being  filled  with  haematite. 
3  Geol.  Mag.  for  1889,  p.  485. 
4  It  has  generally  a  granular  texture,  absence  of  cleavage  and  twinning, 
rhomboidal  form  in  section,  and  high  polarization-tints. 
5  This  is  seen  in  a  Porth-felen  basalt,  and  also  within  the  spheroid  described 
from  Porth-din-lleyn. 
