Yol.  49.] 
CONGLOMERATES  IN  CAERNARVONSHIRE. 
463 
associated  grits,  and  the  concealment  of  the  great  thickness  of  the 
Banded  Slate — I  do  not  see  how  we  can  avoid  the  conclusion  that  the 
conglomerate  lies  unconformably  on,  and  is  independent  of,  the 
underlying  members  of  the  Cambrian  Series. 
IX.  Mynydd-y-Cilgwyn  and  Llanllyeni. 
After  a  certain  interval  from  the  southernmost  exposure  of 
conglomerate  on  Moei  Tryfaen  (about  half-a-mile)  new  deposits, 
obviously  continuing  the  same  line,  come  on  at  Bwlch-y-llyn.  The 
northernmost  seen  are  some  curious  brown-weathering  greenish- 
grey  grits  not  to  be  matched  by  any  known  rock  in  the  Cambrian 
Series,  but  more  like  those  found  at  the  ‘  Silurian  ’  base  in  the 
Bettws  Garmon  Valley.  On  reaching  the  tram-line  cutting  we  find 
that  these  are  underlain  on  the  west,  though  the  line  of  junction 
has  a  high  dip,  by  conglomerate  identical  with  that  of  Moel  Tryfaen 
in  which  fragments  of  purple  slate  are  conspicuous.  East  of  these, 
though  the  contact  is  not  seen,  comes  Purple  Slate.  At  first  sight 
this  section  might  be  taken  to  indicate  a  chronological  succession, 
but  it  is  fairly  (I  might  almost  say  quite)  certain  that  the  latter 
line  of  junction  is  a  fault;  for  to  the  north  there  is  much  more  grit, 
a  little  way  to  the  south  there  is  none,  and  at  J-  mile  to  the  south 
along  the  same  line  the  slate  and  felsite  are  seen  within  a  yard,  the 
former  dipping  towards  the  latter ;  and  half-a-mile  farther  south 
again  the  fault  is  actually  seen  in  the  quarry. 
Now,  if  this  conglomerate  has  to  he  faulted  to  make  it  appear  to 
underlie  the  slate,  its  natural  place  is  somewhere  else ;  and,  as  the 
dip  of  the  slates  indicates  a  downthrow  of  the  felsite,  we  may  con¬ 
clude  that  that  natural  place  is  above  the  slates.  This  is  all  we  can 
learn  with  reference  to  the  slates  ;  but  by  tracing  the  boundaries 
of  the  conglomerate  we  find  that  it  spreads  over  a  wide  and 
irregular  area,  mostly  on  the  northern  and  western  slopes  of 
Mynydd-y-Cilgwyn,  that  it  is  followed  by  banded  grits  on  the  west 
and  north-west,  as  it  was  also  on  the  north-east  at  Bwlch-y-llyn, 
and  that  it  rests  everywhere  on  a  large  continuous  mass  of  felsite. 
In  all  these  points  it  contrasts  absolutely  with  the  conglomerate 
in  the  Moel  Tryfaen  adit,  while  it  agrees  in  character,  succession,  and 
expansion  with  that  at  the  summit,  and  is  distinguished  from  it 
only  by  lying  upon  an  altogether  different  rock.  What  can  such  a 
rock  be  but  an  unconformable  deposit  ? 
The  rock  at  Tal-y-sarn,  brought  forward  as  pre-Cambrian  by 
Dr.  Hicks,  and  described  by  the  late  Mr.  Thos.  Davies  as  a  quartz- 
felsite  breccia,1  is  of  course  nothing  more  than  part  of  the  felsite- 
mass  of  this  area,  which  is  in  places,  as  noted  by  Sir  Archibald 
Geikie,  intensely  cleaved.  Beyond  the  main  mass,  at  a  spot  half-a- 
mile  due  east  of  Llanllyfni,  there  is  a  long,  isolated  boss  of  very 
much  cleaved  quartz-felsite  with  workings  of  Purple  Slate  on  either 
1  Quart.  Journ.  Geol.  Soc.  vol.  xxxiv.  (1878)  p.  152.- 
