Vol.  49-]  CONGLOMERATES  IN  CAERNARVONSHIRE.  457 
upper  band  would  hardly  die  out  above  in  the  solid  rock  nor  be 
separated  by  so  narrow  a  band  of  felsite  from  the  lower ;  nor,  if  it 
filled  cracks  in  the  felsite,  would  these  occur  on  the  upper  side  as 
they  are  seen  to  do.  The  behaviour  of  the  rock  is  rather  that  of  an 
intrusive  one.  Of  the  rock  itself  Prof.  Bonney  says : — “  Microscopic 
examination  shows  the  structure  of  the  slate  to  resemble  that  of  the 
groundmass  of  some  of  the  chiastolite-slates.”  There  is  certainly  a 
superficial  resemblance  to  that  of  the  Skiddaw  chiastolite-slates, 
which,  by  the  kindness  of  Mr.Teall,  I  have  been  able  to  examine  in  the 
Jermyn  Street  collection,  and  at  the  time  of  Prof.  Bonney’s  paper  the 
effects  of  dynamo-metamorphism  on  igneous  rocks  were  not  so  well 
understood  as  they  are  now ;  but  there  can  be  little  doubt  that  this 
6  slate 9  is  merely  an  unusually  squeezed  example  of  an  igneous 
rock  of  fairly  basic  composition.  The  slaty  structure  is  produced 
Fig.  11.  — Intrusive  bands  of  altered  Lamprophyre  (?)  in  the 
Felsite  of  Llyn  Padarn. 
a  ■=  Felsite.  b  =  Altered  Lamprophyre  (?). 
by  innumerable  subparallel  but  interosculating  lines  of  sericite,  and 
gives,  therefore,  no  indication  of  origin.  Eliminating  these,  the 
bulk  of  the  rock  consists  of  much-obscured  lath-shaped  areas  now 
occupied  by  a  minute  mosaic,  but  occasionally  retaining  remains  of 
a  plagioclase,  with  the  interspaces  filled  with  a  green  chloritic  (?) 
material  of  secondary  origin,  and  over  all  are  pretty  uniformly 
dotted  black  specks,  which  are  probably  magnetite.  There  are  also 
occasional  insets,  composed  of  broken  and  separated  fragments  of 
felspar  and  some  infilled  round  cavities.  This  is  the  description  of 
a  diabase,  but  I  am  not  sure  that  the  rock  has  not  been  derived 
from  a  lamprophyre. 
My  reason  for  saying  so  is  that  it  appears  to  belong  to  the  same 
family  as  two  other  rocks  in  the  district.  One  of  them  occurs 
on  the  opposite  side  of  Llyn  Padarn,  almost  in  a  line  with  this  ;  it  is 
quite  as  slaty,  runs  in  the  same  way  into  the  felsite,  and  has  the 
same  general  microscopic  structure,  but  it  shows  a  number  of  large 
Q.  J.  O.  S.  No.  195.  2  h 
