Fig.  12. — Horizontal  Section  along  the  Mod  Tryfaen  adit.  (Horizontal  scale  :  1  inch  =  200  feet.) 
Yol.  49.  J 
CONGLOMERATES  IN  CAERNARVONSHIRE. 
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The  measurements  here  given  -would  be  the 
thicknesses  only  if  the  beds  were  vertical  and 
unfaulted.  They  dip,  however,  at  various  angles 
above  45°.  If  we  take  an  average  of  60°,  this 
would  give  a  total  thickness  below  the  Purple 
Slates  of  1183  feet.  But  there  are  also  several 
breaks  which  may  possibly  be  faults,  as  marked 
b}7  lines  in  the  accompanying  section. 
First,  with  regard  to  the  conglomerate  No.  2, 
it  is  a  very  thin  band — in  fact,  merely  a  minor 
feature  in  a  thicker  band  of  green  grit.  More¬ 
over,  it  lies  at  the  top  of  a  whole  series  of  rocks 
which  can  be  perfectly  correlated  with  well-known 
Cambrian  strata,  and  which  are  not  in  the  least 
more  metamorphosed  than  all  Cambrian  rocks 
are.  It  occupies,  in  fact,  the  position  of  the 
Bhiw-wen  Grit.  Nor  has  it  anything  to  do  with 
any  felsite  •  the  pebbles,  indeed,  are  of  quartzite. 
If,  therefore,  the  conglomerate  on  the  summit 
of  Moel  Tryfaen  were  the  same  as  this,  it  would 
prove  nothing  as  to  any  pre-Cambrian  beds, 
except  their  non-existence  here. 
Next,  with  regard  to  the  felsite.  There  are 
at  least  four  bands  of  this  in  various  parts  of  the 
series.  So  much  we  have  anticipated  from  surface- 
observations  elsewhere,  but  here  all  the  bands 
occur  in  a  single  section,  and  it  is  absolutely 
impossible  to  say  that  they  are  all  at  the  bottom 
of  the  series.  It  is  seen  also  that  they  are  not 
intrusive,  for  one  of  them  lies  conformably  on  the 
surface  of  a  grit,  they  are  for  the  most  part  asso¬ 
ciated  with  tuffs,  and  most  of  the  grits  are 
obviously  formed  of  their  debris.  They  are  not 
at  all  6  metamorphosed/  but  only  cleaved.  They 
are  here,  then,  absolutely  demonstrated  to  be  part 
and  parcel  of  the  Cambrian  Series. 
With  regard  to  the  remainder  of  the  beds, 
their  correlation  is  fairly  obvious  as  marked  in 
the  section.  Attention  may  be  called  to  the 
dyke  of  altered  lamprophyre,  20,  21,  which  has 
been  already  referred  to.  Seeing  that  we  here 
reach  nearly  to  the  base  of  the  Cambrian  suc¬ 
cession,  as  seen  elsewhere,  it  is  a  matter  for 
speculation  whether  the  country  to  the  east  con¬ 
tains  still  lower  beds,  or  whether  part  of  the  same 
succession  has  been  left  behind  in  the  lower 
ground  when  the  other  part  was  pushed  up  into 
this  hill.  The  thick  covering  of  drift  gives  us 
little  opportunity  of  determining  this  point,  but 
the  latter  supposition  is  perhaps  the  most  probable, 
as  the  rocks  seen  at  Groeslon  and  Craig-y-Dinas 
are  not  very  unlike  some  of  the  lower  beds  here, 
and  something  like  Banded  Slates  is  seen  south 
of  the  latter  spot. 
