Vol.  49.]  RADIOLARIAN  CHERT  FROM  MULLION  ISLAND.  213 
One  somewhat  exceptional  variety  may  be  described  as  a  fine¬ 
grained,  dark  greenish-grey  dolerite.  It  is  composed  of  pale  brown 
augite  showing  a  strong  tendency  to  form  long,  slender  prisms, 
lath-shaped  felspars,  and  iron  ores ‘(scarce). 
The  stratified  rocks,  which  form  only  a  very  small  portion  of  the 
island,  consist  of  cherts,  shales,  and  limestones.  They  occur  in  thin 
strips  which  cannot  be  traced  continuously  across  the  island,  and 
are  both  underlain  and  overlain  by  igneous  rocks.  T1  ey  are  seen  at 
different  horizons  from  the  foreshore  to  the  summits  of  the  northern 
and  south-eastern  promontories.  The  chert  is  the  most  interesting 
lithological  type,  on  account  of  its  radiolarian  origin.  It  is  inter- 
stratified  with  shale,  and  occurs  in  bands  which  vary  from  £  inch 
to  several  inches  in  thickness.  As  many  as  thirty  bands  may 
occur  in  a  thickness  of  3  feet.  In  some  places  where  movement 
appears  to  have  taken  place  before  the  final  consolidation  of  the 
cherts,  some  layers  have  been  pinched  out,  while  others  have  been 
thickened  into  nodules.  That  these  nodules  are  not  of  a  concre¬ 
tionary  character  is  shown  by  the  presence  of  radiolaria.  The 
colour  varies  from  black  to  grey  or  brown,  and  very  frequently  the 
thinner  layers  have  a  central  dark  band,  bordered  by  lighter-coloured 
margins.  The  radiolaria  may  often  be  observed  with  a  pocket-lens 
on  the  surfaces  of  cross-joints,  but  they  are  most  perfectly  seen  on 
the  upper  surfaces  of  some  of  the  layers  where  the  shale  has  been  re¬ 
moved  by  the  weather.  Here  they  appear  as  rounded  protuberances, 
often  thickly  crowded  together,  and  their  true  nature  can  be  at  once 
determined  by  placing  such  a  surface  under  the  microscope.  The 
reticulated  character  of  the  test  can  then  be  often  clearly  seen.  The 
chert  bands  are  generally  traversed  by  a  network  of  white  quartz-veins. 
The  shale  varies  in  colour  from  buff  or  brown  to  nearly  black. 
We  have  searched  this  shale  for  fossils,  but  up  to  the  present  time 
only  one  microscopic  form  has  been  detected,  and  that  one  has  not 
been  identified.  The  limestones  are  grey  in  colour  and  crystalline 
in  texture.  Their  precise  relations  to  the  series  of  cherts  and  shales 
cannot  be  very  clearly  made  out.  Hot  unfrequently  they  occur  as 
more  or  less  isolated  lenticles  in  the  greenstone. 
An  excellent  exposure  of  the  rocks  of  the  island  is  seen  in  Tol  Du, 
the  inlet  on  the  southern  side.  The  foreshore  is  formed  of  4  green¬ 
stone,’  and  the  exposed  surface  shows  the  peculiar  appearance  above 
referred  to.  The  more  or  less  distinct  rolls  exhibit  a  tendency  to 
elongation  in  a  north-westerly  and  south-easterly  direction,  and  there 
are  indications  of  a  dip  towards  the  north-east.  Small  patches  of 
chert  and  shale  may  be  seen  sticking  on  this  surface,  and  bits  of 
limestone  occur  between  the  rolls  of  igneous  material.  At  the  base 
of  the  cliff  facing  south-west  is  a  band  of  chert  and  shale  measuring 
a  few  feet  in  thickness.  It  is  covered  by  another  mass  of  rock 
precisely  similar  to  that  which  forms  the  foreshore.  As  the  section 
is  followed  round  the  south-eastern  part  of  the  island  patches  of  the 
chert-shale  series  are  seen  at  different  levels  in  the  igneous  rocks.1 
1  [A  subsequent  visit  has  enabled  me  to  obtain  evidence  that  the  band  of  radio¬ 
larian  chert  traverses  the  third  islet  of  Tregwyn,  as  well  as  the  two  where  it  is 
shown  on  the  map  facing  this  page. — H.  F.,  March  21st,  1893.] 
