214  MESSES.  H.  EOX  AND  J.  J.  H.  TEALL  ON  [May  1 893,- 
The  northern  promontory  also  shows  an  excellent  exposure  of 
the  igneous  and  stratified  rocks.  The  latter  are  hero  from  8  to 
10  feet  thick  at  the  base  of  the  cliff,  where  it  faces  towards  the 
north-west.  The  thickness  rapidly  diminishes  as  the  band  is  followed 
towards  the  south.  The  cherts  are  here  associated  with  black  shalesr 
which  certainly  look  as  if  they  should  yield  fossils.  The  igneous 
rock  possesses  the  same  structure  as  at  Tol  Du,  with  the  same  rude 
N.W.-and-S.E.  strike  in  the  rolled  masses.  The  cherts  and  shales 
are  again  exposed  on  the  upper  surface  of  the  south-eastern  arm 
near  the  central  part  of  the  island,  and  two  bands  may  also  be  seen 
on  the  western  arm.  The  latter  occur  in  the  cliffs  facing  north, 
but  cannot  be  traced  to  the  other  side  of  the  promontory. 
II.  Relation  oe  the  Igneous  and  the  Stkatified  Rocks. 
An  important  question  arises  as  to  the  relation  between  the 
igneous  and  the  stratified  rocks.  Must  the  ‘  greenstone  ’  be  regarded 
as  intrusive  or  contemporaneous  ?  The  fact  that  the  stratified  rocks 
appear  to  have  been  completely  enclosed  in  igneous  material,  and 
that  there  is  no  trace  of  a  floor  over  which  a  lava  could  have  flowed, 
are  points  which  may  be  urged  in  favour  of  the  former  view.  But 
there  are  other  facts  which  are  difficult  to  explain  on  this  hypothesis. 
Thus  the  igneous  rock  is  uniformly  fine  in  grain,  although  de¬ 
veloped  on  a  very  extensive  scale.  There  are  no  marked  signs  of 
metamorphism  in  the  sedimentary  rocks.  The  ropy  structure  is 
quite  unlike  that  of  any  known  intrusive  mass  of  equal  size.  In 
these  circumstances  we  are  tempted  to  ask  whether  the  phenomena 
may  not  be  due  to  the  injection  of  igneous  material  between  the 
layers  of  the  stratified  series  near  the  surface  of  the  sea-bed,  and 
possibly  while  deposition  was  actually  going  on.  In  this  way  thin 
sheets  of  deposit  might  be  detached  and  moved  on  by  the  flow  of 
igneous  material.  The  phenomena  might  be  explained  by  the  simple 
flow  of  a  submarine  lava,  if  such  a  lava  possessed  the  power  of 
insinuating  itself  between  layers  of  deposit  and  tearing  them  up 
during  its  onward  march. 
III.  The  Rocks  oe  the  Keighbottking  Mainland  contkasted  with 
those  oe  the  Island. 
The  rocks  of  Mullion  Island  have  not  been  found  on  any  portion 
of  the  adjacent  coast.  The  opposite  cliffs  are  formed  of  hornblende- 
schist  and  serpentine.  Stratified  rocks  make  their  appearance  at 
Polurrian  Cove,  rather  less  than  a  mile  H.N.E.  of  Mullion  Island. 
The  junction  at  this  point  has  been  described  by  Prof.  Bonney,  in  vol. 
xxxix.  of  this  Journal  (1883),  p.  10.  The  change  from  hornblende- 
schist  to  a  sedimentary  series  consisting  of  dark  slates  with  sandy 
beds  is  abrupt,  and  the  fault  which  hades  to  the  south,  so  as  to 
carry  the  sediments  beneath  the  schists,  is  marked  by  a  breccia.  The 
strike  of  this  breccia,  as  it  is  exposed  on  the  coast,  points  to  Mullion 
Island.  We  endeavoured  to  trace  the  fault  inland  on  the  six-inch 
map,  but,  as  it  does  not  make  a  feature,  and  as  there  are  few 
