Yol.  49.]  RADIOLARIAN  CHERT  FROM  MULLION  ISLAND.  215 
exposures,  this  is  by  no  means  an  easy  task.  The  first  point  where 
the  fault  can  be  fixed  within  a  few  yards  is  at  La  Erowder,  and  if  this 
point  be  joined  to  the  exposure  on  the  coast  and  the  line  continued, 
it  would  pass  to  the  north  of  the  island.  The  trend  of  the  junction- 
breccia  on  the  coast,  therefore,  appears  to  indicate  that  the  fault  curves 
somewhat  towards  the  south.  If  this  curvature  were  continued  for 
a  short  distance,  it  would  carry  the  boundary-fault  between  Mullion 
Island  and  the  mainland,  and  such  is  the  view  we  take.  We  have 
examined  the  coast  from  Polurrian  Cove  to  the  cliffs  no'rth  of 
Gunwalloe  in  the  hope  of  finding  the  rocks  of  Mullion  Island,  but 
without  success.  They  must  apparently  he  looked  for  in  other  parts 
of  Western  Cornwall. 
IY.  Note  on  the  Radiolaria  in  the  Mullion  Island  Chert. 
By  George  Jennings  Hinde,  Ph.D.,  Y.P.G.S. 
[Plate  IV.] 
As  already  mentioned  by  the  Authors,  the  radiolaria  in  this  chert 
are  partially  weathered  out  on  the  surface  of  some  of  the  beds,  and, 
when  examined  under  a  lens  or  under  the  microscope,  they  appear 
like  so  many  millet-seeds,  thickly  covering  the  rock.  In  this 
condition  they  usually  show  the  lattice-like  structure  of  the  test,  of 
a  light  or  dark-brown  tint ;  when  this  has  been  weathered  off,  only 
the  solid  cast  of  translucent  silica  which  has  filled  up  the  originally 
hollow  test  projects  above  the  surface.  The  majority  of  the  forms 
thus  shown  appear  to  be  simple  spheres  belonging  to  the  genus 
Oenosphcerci ,  Ehrenberg  (PI.  IY.  fig.  1.),  but  as  only  the  upper 
portion  of  these  weathered- out  forms  can  be  distinguished,  it  is 
quite  possible  that  some  of  them  may  be  oval  instead  of  spherical. 
In  thin  microscopic  sections  of  the  chert,  the  radiolaria  are  shown 
in  some  portions  nearly  in  contact  with  each  other,  while  in  others 
they  are  less  thickly  distributed.  Most  of  them  appear  as  trans¬ 
parent  bodies  with  circular  or  oval  outlines,  only  marked  off  from 
the  enclosing  matrix  by  the  clearness  of  the  silica  which  has  filled 
in  their  inner  cavities.  The  lattice-structure  of  the  tests  in  these 
forms  has  completely  disappeared,  and  only  the  chalcedonic  casts  of 
the  interior  remain.  Sometimes  the  tests  are  shown  in  section  as 
circular  or  oval  rings  of  a  brownish  tint,  in  which  the  apertures  are 
indicated  by  alternate  lighter  spaces  (PI.  IY.  figs.  4,  5).  In  a  few 
rare  instances  the  tests  have  become  stained  by  an  opaque  dark 
material,  and  these  show  the  structure  fairly  well ;  but  unfortunately 
this  dark  substance  has  often  infilled  the  interior  of  the  tests,  so  that 
only  their  outlines  can  now  be  distinguished.  In  their  general 
condition  of  preservation  the  radiolaria  in  this  Cornish  chert 
strikingly  resemble  those  in  the  Ordovician 1  chert  of  Scotland,  but 
they  are  less  favourably  preserved  in  the  specimens  which  have  as 
yet  been  obtained. 
Owing  to  their  imperfect  preservation,  it  is  not  possible  to  give 
1  Ann.  &  Mag.  Nat.  Hist.  ser.  6,  vol.  vi.  (1890)  p.  40. 
