162 
MISS  C.  A.  RAISIN  ON  VARIOLITE  OF 
[May  1893, 
is  a  banded  halleflinta,  in  which  the  layers  are  developed  by 
weathering.  A  fine-grained  argillaceous  tuff  can  be  seen  near 
Porth  Merin,  and  at  other  places  north  of  it.  These  beds  have  a 
fairly  continuous  dip  to  the  north-west  and  run  out  to  sea  in  low 
points,  while  their  truncated  edges  form  a  flattish  platform,  some¬ 
times  at  the  foot  of  low  grassy  cliff's  of  Drift.  Small  volcanic  centres 
are  perhaps  marked  by  coarse  agglomerates  associated  with  igneous 
masses  (as  at  Pared-llech-y-menyn  and  Maen-melyn-lleyn).1  Else¬ 
where  the  diabase  or  basait,  sometimes  standing  as  an  isolated  reef 
or  islet,  may  have  formed  part  of  the  pipe  or  reservoir  of  a  small 
volcano,  as  at  Dinas-fach,  Dinas-fawr,  and  south  of  Porth  Oer, while 
the  extensive  mass  of  variolite  at  Porth  Orion  probably  is  also  con¬ 
nected  with  a  centre  of  eruption.  Lava-flows  can  be  recognized  in 
green  palagonitic  rocks  (as  at  Careg,  Porth  Oer) ;  also  in  vesicular 
basalt  containing  amygdules  of  calcite,  as  in  a  much-crushed  specimen 
between  Dinas-fach  and  Porth  Oer,  and  in  a  very  coarsely- vesicular 
rock  west  of  Porth  Witlin. 
Yolcanic  rocks,  massive  and  fragmental,  have  thus  been  traced 
from  Bardsey  Island  to  Porth- din-lleyn.  They  appear  on  the  whole 
to  form  one  series,  apart  from  those  intrusions  which,  judging  from 
their  lithological  character,  are  probably  of  post-Ordovician  age.2 
Green  and  purple  layers  among  the  schistose  tuffs  seem  to  be 
homologous  with  the  two  types  of  igneous  rock,  and  the  halleflinta 
of  the  two  southern  hills  is  represented  occasionally  in  interstratified 
bands  of  similar  character.  Grit  and  limestone,  often  in  lenticular 
patches,  varying  from  microscopic  size  to  many  yards  in  length, 
appear  to  be  contemporaneous  with  the  volcanic  series,  although  the 
layers  have  been  subsequently  broken  and  squeezed.  Gradations 
can  be  traced  from  limestone  to  ashy  bands  (as  above  Braich- 
y-pwll),  or  to  a  laminated  basaltic  mud,  as  if  the  finer  volcanic 
debris  was  washed  into  calcareous  waters.  At  Porth-din-lleyn,  as 
I  have  shown,3  small  igneous  fragments  appear  to  be  embedded. 
Limestone  is  also  included  in  large  blocks  within  the  basalt,  and  in 
these  and  other  examples  the  microscopic  slide  consists  of  very 
regular,  uniform  rhombohedra,  suggestive  of  contact-alteration.  It 
resembles  the  structure  shown  to  me  by  Prof.  Bonney  in  a  piece  of 
Trenton  limestone,  which  had  been  caught  up  in  a  dyke  at  Corpora¬ 
tion  Quarry,  Montreal.  The  grit  consists  mainly  of  quartz  and  of 
felspar,  often  plagioclase,  sometimes  microcline.  It  occasionally 
contains  flakes  of  chlorite  or  mica,  fragments  of  mica-schist  or  of 
a  compact  scoriaceous  or  igneous  rock  ;  and  a  grain  of  tourmaline 
occurs  in  one  slide.  The  grit  has  evidently  been  derived  partly 
from  old  granitoid  rock  and  schists,  and  thus  its  accumulation  must 
1  At  this  place,  where  the  streamlet  falls  into  the  sea,  is  St.  Mary’s  Well,  of 
ancient  repute.  The  trough-shaped  pool  of  clear  water,  wherein  grow  delicate 
green  algae,  is  hollowed  out  at  the  foot  of  a  vertical  cliff  of  coarse  agglomerate. 
The  water  is  fresh  and  sweet,  although  the  surface  is  a  very  few  feet  above  the 
level  of  a  calm  sea  at  high  tide. 
2  G-eol.  Mag.  for  1892,  p.  409. 
3  Ibid.  p.  410. 
