Vol.  49.] 
RADIOLARIAN  CHERT  FROM  MULLION  ISLAND. 
211 
15.  On  a  Kadiolarian  Chert  from  Mullion  Island.  By  Howard 
Box,  Esq.,  F.G.S.,  and  J.  J.  H.  Teall,  Esq.,  M.A.,  F.R.S., 
F.G.S.  (Bead  February  8th,  1893.) 
Contents. 
Page 
I.  Description  of  the  Island  and  its  Predominant  Rocks .  211 
II.  Relation  of  the  Igneous  and  the  Stratified  Rocks  .  214 
III.  The  Rocks  of  the  Neighbouring  Mainland  contrasted  with  those  of 
the  Island .  214 
IV.  Note  on  the  Radiolaria  by  Dr.  G.  J.  Hinde  (Plate  IV.)  . .  215 
I.  Description  of  the  Island  and  its  Predominant  Bocks. 
Mullion  Island  lies  off  the  western  coast  of  the  Lizard  Peninsula, 
a  little  south  of  Mullion  Cove.  The  channel  separating  it  from 
the  mainland  is  only  about  250  yards  wide  at  its  narrowest  part, 
and  yet  the  rocks  of  which  the  island  is  composed  are  quite  dis¬ 
tinct  from  those  of  the  adjacent  coast. 
The  island  somewhat  resembles  in  shape  the  leaf  of  the  trefoil. 
From  the  central  part  three  promontories,  each  150  yards  in  length, 
answering  to  the  three  lobes  of  the  leaf,  run  out  into  the  sea ;  one 
towards  the  north,  another  towards  the  west,  and  a  third  towards 
the  south-east.  The  coast-line  is  almost  everywhere  backed  by 
cliffs  which  vary  in  height  from  20  to  100  feet.  The  culminating 
point  lies  at  the  extreme  edge  of  the  western  promontory,  facing 
the  open  sea.  The  upper  portions  of  the  cliffs  in  this  part  of  the 
island  glow  with  brilliant  orange  and  yellow  tints,  owing  to  the 
luxuriant  growth  of  the  ubiquitous  lichen  Parmelia  jparietina. 
The  lower  part  of  the  island  is  covered  in  places  with  miniature 
forests  of  tree-mallow.  Sea-beet  and  other  marine  plants  abound. 
The  predominant  rock  is  of  igneous  origin.  It  is  greenish,  fine¬ 
grained,  much  jointed,  and  often  highly  decomposed.  The  old  term 
c  greenstone  ’  may  be  appropriately  applied  to  it.  In  the  mass  it  is 
separated  into  rude  rolls  by  curvilinear  joints.  These  rolls  show 
circular  or  elliptical  outlines  in  cross-section  and  measure  from  a 
few  inches  to  2  feet  in  diameter.  Flat  surfaces  of  this  rock, 
such  as  are  exposed  in  many  places  at  the  base  of  the  cliff,  remind 
one  somewhat  of  the  appearance  of  a  lava  of  the  jpahoehoe  type. 
A  few  spherical  amygdaloids  are  scattered  through  the  mass. 
Under  the  microscope  the  common  type  of  rock  is  seen  to  consist  of 
felspar,  augite,  iron  ores,  and  secondary  products,  such  as  carbonates 
and  chlorite,  derived  from  these  minerals.  The  felspar  may  occur 
as  porphyritic  crystals  sparsely  scattered  through  the  groundmass, 
or  as  long,  slender  microlites  forming  a  constituent  of  the  ground- 
mass.  The  augite  occurs  as  small  grains  and  microlites,  and  is  some¬ 
times  only  represented  by  secondary  minerals.  Iron  ores  are  com¬ 
paratively  scarce.  In  structure  the  rock  is  more  allied  to  a  lava 
than  to  an  intrusive  mass.  Small  veins  filled  with  calcife  are  very 
common. 
p  2 
