Yol.  49.] 
CGN&LOIIERATES  IN  CAERNARVONSHIRE. 
443 
some  being  dark  purple  in  colour.  Both  varieties  are  massive  in  some 
parts,  and  in  others  cleaved  and  drawn  out  into  a  slate-like  form. 
These  felsites  are  followed  towards  the  S.E.  by  a  band  of  fine  green 
grit,  which  (neglecting  dykes)  becomes  still  finer  till  it  is  almost  a 
slate,  light  green,  and  very  obscurely  banded.  Then  comes  (after 
another  dyke)  a  band  of  peculiar  rock  called  ‘  whetstone  It  is 
green  and  translucent,  and  of  course  extremely  fine  in  grain.  I 
have  not  analysed  it,  but  would  suggest  that  it  may  be  composed  of  the 
finest  felsitic  dust.  Next  in  succession  after  another  dyke  is  a  band 
of  conglomerate  whose  matrix  is  a  green  grit,  and  whose  pebbles, 
which  are  not  numerous  and  are  coated  with  sericite,  are  of  red 
felsite,  not  like  any  of  the  samples  obtained  in  situ.  Immediately 
after  this  is  the  ‘  Hard  Blue  ’  slate,  which  continues  to  the  entrance 
of  the  tunnel. 
The  second  shaft  is  not  yet  completed.  It  commences  in  the 
whetstone,  and  after  a  considerable  depth,  the  dip  of  the  strata  being 
high,  it  reaches  the  grit,  and,  passing  through  this,  enters  the  felsite, 
in  which  it  is  likely  to  end.  East  of  this  we  find  on  the  surface 
skirting  the  edge  of  the  quarry  the  green  grit  and  conglomerate 
of  the  tunnel.  The  pebbles  are  seen  to  be  scarce  and  confined  to 
narrow  bands  ;  per  contra,  they  are  of  large  size  up  to  the  diameter 
of  a  soup-plate.  Thus  the  section  in  the  tunnel  is  confirmed. 
How,  in  this  case,  we  know  exactly  where  we  are  in  the  series  ; 
for  the  workings  of  the  quarr}T  lay  bare  all  the  strata  up  to  the 
Bronllwyd  Grit.  Succeeding  the  Hard  Blue  comes  the  Bed  Band,  then 
the  Old  Blue,  and  above  that  the  Bastard  and  Pale  Slates.  Under 
ordinary  circumstances  I  should  dismiss  this  section  with  the  remark 
that  the  felsite  and  conglomerate  are  on  the  horizon  of  the  St.  Ann's 
Grit,  and  simpty  point  out  that  the  last-named  is  thus  shown  to  be 
occasionally  conglomeratic.  But  if  the  matter  has  to  be  argued  out, 
I  must  say  that  we  are  here  left  with  a  triple  alternative — either 
(1)  the  workable  slates  of  the  Penrhyn  Quarry  are  the  only  rocks 
which  can  be  rightly  called  Cambrian,  so  that  this  conglomerate  lies 
at  the  base  of  that  system  ;  in  this  case  all  the  other  conglomerates 
would  have  to  be  pre-Cambrian,  though  they  also  have  been  claimed 
as  basal  Cambrian  ;  or  (2)  the  beds  to  the  west  of  this  spot  are 
repetitions  of  the  Penrhyn  Slates  in  another  form,  as  is  claimed  to 
be  the  case  by  Hr.  Hicks,  in  the  district  a  little  farther  south ;  in 
this  case  we  have  beds  which  remain  constant  for  15  miles  in  a 
H.E.-and-S.W.  direction,  and  even  appear  again  in  a  recognizable 
form  in  the  centre  of  the  Harlech  anticlinal,  suddenly  changing 
their  character  in  14  mile  in  a  north-westerly  direction,  so  as  to 
retain  no  resemblance  whatever  to  their  old  form ;  or  (3)  if,  as  I 
hold,  both  the  above  alternatives  are  to  be  rejected,  the  beds  to  the 
west  are  lower  ones  in  the  Cambrian  succession  and  the  felsite  is  a 
mass  poured  out  or  intruded  in  the  middle  of  the  Cambrian  period, 
and  the  succeeding  conglomerate  is  not  at  the  base  of  the  Cambrian. 
In  the  above  section  no  question  arises  as  to  any  later 
conglomerate ;  but  before  leaving  this  district  it  may  be  well, 
for  the  sake  of  comparison,  to  call  to  mind  the  phenomena 
2  g  2 
