146 
MISS  C.  A.  RAISIN  ON  VARIOLITE  OF 
[May  1893, 
agglomeratic,  but  include  crushed  examples  of  igneous  origin.  The 
exact  nature  of  some  rocks,  however,  even  when  illustrated  by 
microscope  slides,  is  difficult  to  determine  precisely.  I  have  been 
obliged  also  to  leave  the  geological  age  of  the  variolites  an  open 
question,  since  even  the  associated  limestone,  quartzose  rocks,  and 
others  which  are  possibly  sedimentary,  give  no  trustworthy  evi¬ 
dence  bearing  upon  this  point. 
As  much  of  my  work  was  done  at  University  College,  London,  I 
had  the  advantage  of  submitting  doubtful  points  to  Prof.  Bonney, 
and  have  to  thank  him  for  valuable  advice  and  assistance. 
II.  Lithological  Characters  of  the  Igneous  Rocks. 
Among  the  holocrystalline  rocks,  some  at  least  are  most  probably 
intrusions  belonging  to  a  later  period.  The  well-defined  dolerite  of 
Trefgraig,  Hendrefor,  Tyhen,  and  Methlan  is  one  type  1 ;  this  has 
been  already  described,  and  very  similar  to  it  is  a  greenish  diabase 
forming  the  headland  of  Trwyn  Glas ;  certain  other  green  porphy- 
ritic  rocks  may  be  related,  although  they  are  not  identical.2  A 
gabbro  is  found  east  of  Forth  Witlin  which  perhaps  is  another  pro¬ 
trusion  of  the  Craig-y-fael  mass  described  by  Mr.  Harker.3  It 
consists  of  plagioclase  felspar,  much  decomposed,  and  of  diallage, 
sometimes  connected  in  an  ophitic  manner.  The  rock  has  been 
more  or  less  affected  by  pressure,  as  even  the  most  normal  specimen 
exhibits  bending  of  the  cleavage-planes  and  other  signs  of  strain, 
while  much  of  the  mass  is  markedly  schistose.  The  dykes  along  the 
shore  vary  somewhat  in  character,  but  many  are  a  compact  diabase 
with  fluidal  structure  ;  4  microporphyritic  felspars  are  crowded  near 
the  edge,  ranging  roughly  parallel,  as  if  they  had  been  floated  to 
the  side.5 
The  igneous  rocks  which  remain  to  be  considered  appear  from  the 
microscopic  characters  to  have  a  certain  uniformity  in  composition, 
and  to  belong  to  the  class  of  rather  basic  andesites  or  not  very  basic 
basalts ;  but  it  is  not  always  easy  to  decide  how  far  we  are  dealing 
with  two  distinct  magmas,  or  with  one  lava  in  which  subsequent 
1  J.  V.  Elsden,  Geol.  Mag.  for  1888,  p.  304  ;  0.  A.  Raisin,  ibid.  1892,  p.  409  ; 
also  Alfred  Harker,  ‘  The  Bala  Volcanic  Series,  etc.’  p.  87. 
2  A  diabase  at  Careg  contains  white  augite ;  see  Geol.  Mag.  for  1892,  p.  412. 
Others  occur  at  Porth-din-lleyn,  and  at  the  beach  between  Dinas-fach  and 
Porth  Oer. 
3  Quart.  Journ.  Geol.  Soc.  vol.  xliv.  (1888)  pp.  447,  448;  and  ‘The  Bala 
Volcanic  Series,  etc.’  pp.  89-92. 
4  See  ‘The  Bala  Volcanic  Series,  etc.’  p.  Ill;  also  J.  F.  Blake,  Quart. 
Journ.  Geol.  Soc.  vol.  xliv.  (1888)  p.  531.  As  Mr.  Harker  states,  it  cannot  be 
proved  that  these  dykes  are  of  the  age  to  which  he  inclines  to  attribute  them, 
but  I  fail  to  understand  Mr.  Blake’s  suggestion  that,  if  contemporaneous  with 
the  volcanic  ashes,  they  are  in  some  cases  ‘  actual  flows,’  in  others  ‘  due  to  infil¬ 
tration.’ 
0  Compare  Quart.  Journ.  Geol.  Soc.  vol.  xlvi.  (1890)  p.310.  A  felstone-dyke 
occurs  near  Porth  Meudwy,  which  is  lithologically  similar  to  the  felstones  of 
Pwllheli.  This  dyke,  however,  is  to  the  eastward  of  the  boundary-fault,  and 
cuts  the  black  shales,  which  are  probably  of  Ordovician  age. 
