Tol.  49.]  THE  LLETH,  AX'D  ASSOCIATED  VOLCANIC  ROCKS.  147 
differentiation  has  taken  place.  Of  the  two  extreme  types,  the  one 
is  a  dull-greenish,  compact  rock,  the  groundmass  including  lath¬ 
shaped  microliths  of  felspar,  accompanied  tty  more  or  less  viridite, 
opacite,  and  minute  granular  augite  or  epidote.  Generally  where 
the  viridite  is  an  important  constituent,  the  iron  oxide  and  the 
augite  are  absent  or  small  in  quantity.  Certain  more  compact  rocks 
(consisting  mainly  of  palagonite  or  chlorite)  were  probably  originally 
glassy,  and  in  some  the  traces  of  a  ffuidal  structure  are  still  retained. 
(See  PI.  I.  fig.  3.)  One  slide  contains  rich  brown,  isometric 
crystals,  often  imperfect,  probably  an  early  formation  of  chromite  or 
picotite.1  The  second  type  of  rock  is  best  illustrated  in  a  variety, 
the  microscopic  characters  of  which  seem  to  justify  the  designation 
of  basalt  or  ferruginous  basalt.  (PI.  I.  figs.  1,  2,  5.)  It  is  compact, 
of  a  dull  reddish  colour,  and  consists  of  opacite  or  ferrite  separating 
lath-shaped  microliths  of  felspar.  These  are  not  infrequently  pseudo- 
morphosed,  so  that  the  original  composition  of  the  mass  must  have 
undergone  alteration.  The  two  types  seem  to  be  connected,  for  one 
and  the  same  rock  may  show  a  development  of  each  kind.2  Both 
forms  also  exhibit  spherulitic  growth,  and  include  similar  porphy- 
ritic  or  microporphyritic  crystals.  Some  of  these  are  felspar, 
others  a  serpentinized  mineral,  the  form  and  general  aspect  of  which 
suggest  a  ferro-magnesian  silicate,  probably  bastite,3  and  there 
seem  to  be  two  varieties  present,  the  one  being  crossed  and  out¬ 
lined  by  bars  of  granular  opacite.  Microliths  of  a  similar  nature 
occur,  either  yellowish  or  pale  greenish,  with  a  thick  border  ot 
iron  oxide.  They  are  acicular  or  shuttle-shaped,  such  as  have 
been  often  figured.4 
Thus  petrological  study  suggests  that  the  rocks  represent  two 
varieties,  with  only  slight  differences  in  composition  and  structure, 
and  that  they  were  probably  formed  by  differentiation  in  a  magma 
originally  homogeneous.  Owing  to  this,  the  ferrite  may  have  crys¬ 
tallized,  and  consolidation  have  begun,  in  one  mass  rather  earlier 
than  in  the  other.  This  view  seems  to  accord  with  the  field  evidence. 
While  in  certain  parts  the  green  diabase  or  dark  ferruginous 
basalt  can  be  easily  recognized,  elsewhere  they  are  more  or  less 
intermingled,  and  even  cannot  be  separated.  Thus  a  mottled  red 
and  green  rock  (at  Porth  Orion)  appears  to  have  been  formed  by  a 
second  magma  intruding  into  the  mass  of  diabase  before  it  was 
solidified.  Also  a  junction  of  two  varieties  within  the  heart  of  a 
spheroid  proves  that  these  were  distinguished  before  the  cooling  and 
contraction  were  complete.5  As  will  be  presently  described,  frag- 
1  The  crystals  are  too  small  to  allow  of  the  hardness  being  tested. 
2  As  in  the  green  and  purplish  rock  from  west  of  Hendre-uchaf,  see  Geol. 
Mag.  for  1892,  p.  412. 
3  Fouque  et  Michel-Levy,  ‘Mineralogie  Micrographique,’  1879,  pi.  xxvi. 
figs.  1,  2.  &  pi.  xxvii. 
4  U.S.  G-eol.  Explor.  of  the  40th  Parallel,  vol.  yi.  ‘Micr.  Petrogr.’  1876, 
F.  Zirkei.  pi.  i.  fig.  20 ;  J.  J.  H.  Teall,  ‘  Brit..  Petrogr.’  p.  14,  fig.  5  (after 
Zirkel) :  F.  Rutley,  ‘Aotes  on  Crystallites,’  Min.  Mag.  vol.  ix.  (1891),  p.  268, 
fig.  17  (Crenulites). 
5  This  junction  was  examined  microscopically. 
L  2 
