Vol.  49.] 
THE  LLEYN,  AND  ASSOCIATED  VOLCANIC  ROCXS. 
163 
have  been  separated  from  their  formation  by  a  long  interval  of  time, 
such  as  everywhere  parts  the  old  Archaean  from  Pebidian  or  early 
Palaeozoic  rocks.1 
The  question  of  the  geological  age  of  these  rocks  is  extremely 
difficult.  Lithologically  some  of  them  resemble  the  Pebidian  s  of 
Caernarvonshire  or  the  northern  district  of  Anglesey.  At  one  place 
I  found  two  badly  preserved  fossils  in  an  indurated  argillite,2  which 
reminded  me  of  Ordovician  strata  ;  but  this  may  be  a  case  of  inter¬ 
folding,  and  thus  it  gives  no  absolute  proof  of  the  age.  I  submitted 
the  rock  to  Dr.  Hicks,  who  kindly  gave  me  his  opinion,  that  it 
seemed  “  to  belong  to  the  Arenig  Series,”  3  and  that  the  organisms 
resembled  phyllopod  crustaceans.4  Unless  further  details  should  be 
obtained,  we  must  leave  undecided  the  geological  position  of  these 
volcanic  rocks,  and  the  question  whether  they  belong  to  very  late 
Archaean  or  to  early  Palaeozoic  time.  But  the  uncertainty  hardly 
seems  to  justify  us  in  erecting  a  new  system  to  contain  these  and 
other  similarly  dubious  masses. 
The  stratigraphical  interest  of  the  area  consists  in  its  volcanic 
characters.  Basaltic  lavas  occur,  of  which,  as  Mr.  Harker  points 
out,  no  other  examples  have  been  described  in  Caernarvonshire ; 5 
and  a  variolite  is  connected  with  contraction-spheroids,  as  in 
German  and  Alpine  localities.  The  cliffs  and  low  crags  along  the 
coast  exhibit  clear  dissections  of  volcanic  structures.  Some  igneous 
rocks  seem  to  mark  the  remains  of  the  reservoir  or  the  vents  from 
which  lava  flowed  or  fragments  were  ejected  ; 6  but  most  of  the 
widely  extended  ashy  deposits,  especially  the  finer  ashes,  have 
probably  come  from  more  distant  centres.  The  igneous  masses  and 
coarser  agglomerates  in  the  south-west  and  at  places  along  the  coast 
suggest  that  important  vents  were  probably  situate  in  parts  now 
covered  by  the  sea.  The  volcanic  formations  seem  to  have  accumu¬ 
lated  near  a  shore-line,  where  limestones  were  formed  in  shallow 
lagoons,7  and  where  the  waters  deposited  grit  and  mud,  derived 
partly  from  ancient  granitoid  rock,  partly  from  later  volcanic 
materials. 
1  In  several  places,  Prof.  Bonney  has  pointed  out  the  contrast  shown  by  these 
Archaean  and  later  series  of  rocks. 
2  Associated  with  the  volcanic  rocks  of  Pared-lleck-y-menyn  on  the  south 
coast. 
3  Dr.  Hicks  reported  that  this  specimen  and  two  from  another  locality  were 
i  probably  of  Llanvirn  age  like  the  Pont  Seiont  Beds  at  Caernarvon.’ 
4  The  fossils  were  afterwards  submitted  to  Prof.  T.  Bupert  Jones,  F.B.S., 
who  has  kindly  favoured  me  with  the  note  appended  to  the  present  paper, 
p.  164. 
5  4  The  Bala  Yolcanic  Series,  etc.,’ 1889,  p.  75.  Mr.  Harker  points  out  that 
the  rocks  at  Porth-din-lleyn,  previously  described  by  Prof.  Bonney,  might 
include  such  examples.  See  papers  quoted. 
6  The  fine-grained  clastic  rock  near  the  spring,  north  of  Mynydd  Annelog, 
possibly  represents  material  thrown  out  from  the  small  vent  to  the  northward, 
since  the  constituents  are  similar,  except  in  size.  See  Geol.  Mag.  for  1892, 
p.  413. 
7  The  possibility  of  precipitation  of  carbonate  of  lime  in  shallow  pools  among 
volcanic  detritus  on  land,  as  at  the  well-known  Temple  of  Serapis,  must  not  be 
overlooked. 
