Yol.  49.] 
RADIOLARIAN  ROCK  FROM  AUSTRALIA. 
221 
16.  Note  on  a  Radiolarian  Rock  from  Fanny  Bay,  Port  Darwin, 
Australia.  By  George  Jennings  Hinde,  Ph.D.,  Y.P.G.S. 
(Read  February  8th,  1893.) 
[Plate  V.] 
About  two  years  ago,  Capt.  Moore,  of  H.M.S.  ‘  Penguin,’ 
brought  home  to  this  country  and  forwarded  to  the  Admiralty  a 
block  of  a  white  rock  obtained  from  the  cliffs  at  Fanny  Bay,  Port 
Darwin,  in  the  Northern  Territory  of  the  Colony  of  South  Australia. 
The  specimen  was  forwarded  by  Capt.  W.  J.  Wharton,  F.R.S.,  the 
Hydrographer  to  the  Admiralty,  to  Sir  Archibald  Geikie,  For.  Sec. 
R.  S.,  Director-General  of  the  Geological  Survey,  who,  with  Capt. 
Wharton’s  consent,  kindly  allowed  me  to  make  a  microscopical 
examination  of  it,  and  the  results  seem  of  sufficient  interest  to  com¬ 
municate  to  the  Society. 
The  rock  in  question  is  of  a  dull- white  or  yellowish- white  tint,  in 
places  stained  reddish  with  ferruginous  material ;  it  has  an  earthy 
aspect,  like  that  of  our  Lower  White  Chalk,  but  it  is  somewhat 
harder  than  chalk,  though  it  can  be  scratched  with  the  thumb-nail. 
There  are  no  signs  of  stratification,  and  it  appears  as  a  fine-grained, 
homogeneous  material.  Unlike  chalk,  however,  it  gives  no  reaction 
either  in  cold  or  heated  hydrochloric  acid.  When  thoroughly  dry 
it  readily  breaks  up  into  flakes  with  uneven  surfaces.  Though 
somewhat  soft,  thin  microscopical  sections  of  it  can  be  prepared 
without  much  difficulty,  and  these  show  a  fairly  transparent  ground- 
mass  containing  numerous  very  minute  granules  and  subangular 
mineral-fragments  ranging  up  to  '075  mm  in  breadth.  The  ground- 
mass  itself  is  quite  negative  in  polarized  light,  and  appears  to  be 
made  up  of  amorphous  silica,  but  the  minute  grains  and  angular 
particles  with  which  it  is  filled  readily  polarize,  and  some  of  them 
appear  to  be  of  quartz,  while  others  are  probably  rutile. 
Besides  these  microliths,  the  rock  contains  numerous  small  cir¬ 
cular  and  elongate  bodies  of  a  clearer  aspect  than  the  matrix, 
owing  to  the  fact  that  the  silica  composing  them  is  largely  free 
from  the  mineral  particles.  These  bodies  are  very  irregularly 
distributed  through  the  rock ;  in  some  portions  they  are  so  crowded 
as  to  be  in  contact  with  each  other,  while  in  others  they  are  sparsely 
scattered  here  and  there.  The  greater  number  of  these  bodies 
show  nothing  more  than  mere  outlines ;  in  a  few  the  structural 
details  are  preserved,  though  in  a  very  faint  and  imperfect  manner, 
yet  these  are  sufficient  to  prove  that  the  organisms  are  radiolaria, 
and  that  the  rock  is  really  a  radiolarian  earth,  intermediate  in 
character  between  soft,  incoherent,  radiolarian  material  (like  that 
from  the  Tertiary  strata  of  Barbados)  and  compact  chert  (like 
that  from  the  Ordovician  strata  of  the  South  of  Scotland  and  that 
described  by  Messrs.  Fox  and  Teall  from  Mullion  Island,  Cornwall). 
