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KEY.  J.  E.  BLAKE  ON  EELSITES  AND 
[Aug.  1893, 
black-looking  scales,  which  are  really  plates  of  chlorite,  and  there  are 
many  smaller  specks  of  the  same.  The  other  is  a  massive  rock,  to 
be  presently  noted  as  occurring  in  the  adit  of  Moel  Tryfaen.  This 
is  built  in  the  same  way,  hut  its  original  structure  is  not  so  much 
obliterated,  and  it  is  uniformly  speckled  with  small  scales  of  chlorite 
lying  parallel  to  each  other,  thereby  producing  so  close  a  resemblance 
to  specimens  of  lamprophyre  in  my  collection  that,  considering  the 
chlorite  is  doubtless  secondary,  it  may  well  have  been  derived  from 
such  a  rock.  If  the  rock  in  hulk  was  a  lamprophyre  we  ought,  I 
should  suppose,  to  call  the  tongues  of  it  by  the  same  name,  though 
this  particular  one  shows  no  chlorite-scctZtfs. 
VII.  Bettws  Garmon  Valley. 
On  one  side  of  this  valley  we  have  the  large  and  complex  mass 
of  felsite  forming  Moel  Smythaw,  and  on  the  other  the  tiny  boss  of 
Gareg-fawr.  I  have  to  thank  Mr.  Marr  for  calling  attention,  in  the 
discussion  of  my  former  paper,  to  the  occurrence  of  conglomerate  in 
connexion  with  these  masses,  for  I  had  certainly  missed  it.  There 
are  large  crags  of  it  under  Moel  Smythaw,  due  west  of  Bettws 
Garmon ;  the  south-eastern  side  of  Gareg-fawr  is  also  conglomerate, 
and  the  line  joining  these  is  parallel  to  the  junction  of  the  Purple 
Slates  with  the  overlying  4  Silurian.’ 
There  can  be  no  question  that  anyone  seeing  this  succession  alone 
would  consider  it  self-evident  that  the  felsite,  conglomerate,  and 
slate  are  in  chronological  order.  It  is  not,  indeed,  impossible  that 
they  should  be.  The  conglomerate  would  then  correspond  to  that 
seen  in  the  Penrhyn  Quarry  tunnel,  and  in  the  adit  of  Moel  Tryfaen, 
only  here  there  is  no  associated  green  grit  that  I  could  find.  On 
the  other  hand,  the  ‘  Silurians  ’  (which  are  only  1000  feet  away) 
may  have  formerly  overlapped  as  far  as  here.  They  would  then 
have  to  be  brought  down  by  a  fault,  just  as  we  find  has  happened 
farther  south,  along  a  line  which  is  a  continuation  of  the  line  of 
junction  here.  And  this  seems  to  me  the  most  likely  interpretation 
of  what  we  see. 
VIII.  Moel  Tryfaen. 
It  would  be  scarcely  too  much  to  say  that  the  structure  of  Moel 
Tryfaen  is  up  to  the  present  time  entirely  unknown.  It  is  worse 
than  unknown — it  is  wrongly  imagined.  Had  my  own  account  of 
it  been  published  last  year  it  would  not  have  been  correct.  The 
section  in  the  adit  is  a  very  complete  one,  and  is  quite  different  from 
what  had  been  supposed.  It  is  curious  that  this  adit,  which  was  the 
spot  where  pre-Cambrian  rocks  were  first  supposed  to  be  seen  in 
this  part  of  Caernarvonshire,  should  be  also  the  spot  that  is  calcu- 
