160 
YARIOLITE  OF  THE  LLEYX. 
[May  1893. 
VIII.  Stratigraphical  Summary  of  the  District. 
For  study  of  the  rocks  in  the  field  we  are  limited  generally  to 
exploration  of  the  coast.  Only  rarely  is  any  mass  exposed  inland 
above  the  surface  of  the  drift,  and  most  of  such  isolated  outcrops 
were  marked  as  the  so-called  ‘  serpentines.’ 1  (See  Map.) 
My  examination  of  Bardsey  Island  was  somewhat  hurried,  and  I 
did  not  explore  the  cliffs  along  its  north-eastern  coast,  but  its 
rocks  are  evidently  similar  to  those  in  the  neighbouring  parts  of 
the  mainland.  From  a  boat  a  good  view  is  obtained  of  the  coarser 
agglomerates  (including  large  compact  blocks  and  streaky  lateiite) 
which  form  the  eastern  cliffs  north  of  Yr-henllw}n.  The  lower  part 
of  the  island  to  the  southward,  on  which  the  lighthouse  is  built, 
consists  of  well-banded,  ashy  rocks,  sometimes  fine-grained,  with 
associated  quartzose  and  calcareous  masses.  This  series  apparentl) 
forms  the  lowest  strata  exposed,  since  the  dip  is  fairly  steady  over 
most  of  the  island,  as  marked  on  the  Geological  Survey  map.  The 
extreme  southerly  point  exhibits  agglomerate,  and  also  a  purple 
basaltic  tuff  and  a  limestone  included  within  a  green  diabase.  At 
three  places  I  found  a  coarsely  crystalline  rock,  more  or  less  crushed, 
probably  intrusive  and  allied  to  the  granitoid  mass  near  Blangwnadl. 
The  most  northerly  of  these  includes  a  green  mineral  which  is 
probably  an  altered  biotite,  while  in  the  next  example  the  compact 
pinkish  mass  is  interrupted  by  large  plates  of  silvery  mica,  doubtless 
a  primary  constituent  subsequently  modified.  The  third  rock  (on 
the  west  coast)  has  a  wavy  and  interrupted  lamination  probably 
caused  by  pressure,  the  results  of  which  can  be  recognized  also  in 
the  microscopic  section.  Patches  of  a  greenish,  vermiform  chlorite 
(doubtless  prochlorite)  extend  in  thin  strings  among  the  giains  of 
quartz,  which  present  the  appearance  of  having  been  crushed  and 
redeposited. 
On  the  mainland,  the  eastern  boundary-fault  at  Parwyd  Cave 
brings  down  strata,  probably  of  Ordovician  age,  against  the  contorted 
quartzose  and  micaceous  schists  of  Mynydd  Cristion.  These  have 
the  appearance  of  an  ancient  series,  and  are  probably  related  to  the 
schists  of  Anglesey.  Since  similar  types  are  found  on  Mynydd 
Ystum,3  it  is  possible  that  an  axis  of  Archaean  rock  strikes  roughly 
north-eastward  near  the  boundary-fault.4  Along  the  south  coast, 
westward  of  the  schists,  are  rocks  which  appear  to  belong  to  the 
volcanic  series.  Some  at  least  seem  to  be  tuffs.  The  cliffs  west¬ 
ward  and  northward  most  largely  consist  of  ashy  and  agglomerate 
strata,  often  schistose,  and  including  grit  and  limestone  (see  Map). 
Over  the  hills  of  Mynydd-y-gwyddel  and  Mynydd  Annelog,  the  rock 
1  T.  G.  Bonney,  ‘  On  the  Serpentine  and  Associated  Bocks  of  Anglesey  ;  with 
a  Note  on  the  so-called  Serpentine  of  Porth-din-lleyn,’  Quart.  Journ  Geol. 
Soc.  vol.  xxxv ii.  (1881)  pp.  10-50.  See  Geol.  Mag.  for  1892,  p.  408,  and 
literature  there  quoted.  noQQ.  aaaaar 
2  Alfred  Harker,  Quart.  Journ.  Geol.  Soc.  vol.  xliv.  (1888)  pp.  Ill  lib. 
3  J.  F.  Blake,  ibid.  p.  533.  ,  ..  , ,,  . 
4  I  found  near  Tociau  a  similar  specimen  ;  it  was  loose,  but  close  to  the  debris 
thrown  up  from  the  shaft  of  a  small  boring  made  in  search  of  coal  ! 
