222  DR.  G.  J.  HINDE  ON  A  RADIOLARIAN  ROCK  FROM  [May  1 893, 
Though  there  is  a  considerable  difference  between  the  outer  aspect  of 
this  earthy  rock  and  that  of  the  older  chert,  a  microscopical  exami¬ 
nation  shows  an  unexpected  resemblance  both  in  the  character  and 
condition  of  the  minute  organisms,  and  in  the  nature  of  the  matrix. 
The  following  is  a  chemical  analysis  of  this  white  rock,  for  which 
I  am  indebted  to  the  kindness  of  Mr.  A.  Dick,  Jun.  It  has  been 
executed  in  the  Laboratory  of  the  Geological  Survey  : — 
Per  cent. 
84-20 
Trace 
10-70 
not  determined. 
5-00 
99-90 
Unfortunately,  details  of  the  extent  of  the  deposit  whence  this 
radiolarian  rock  was  obtained,  and  its  relationship  to  the  other 
rocks  of  the  country,  are  as  yet  unobtainable.  The  only  published 
information  respecting  the  rocks  of  the  district,  which  I  can  find, 
is  contained  in  a  Deport  by  the  late  Dev.  J.  E.  Tenison- Woods,1 
in  which  it  is  stated  that  nearly  all  the  cliffs  in  the  Northern  Terri¬ 
tory  or  Arnheim’s  Land,  as  it  is  called,  are  capped  by  beds,  from 
16  to  over  100  feet  in  thickness,  of  a  compact,  white  or  yellowish- 
white,  sometimes  ferruginous  rock,  which  is  stated  to  be  for  the 
most  part  magnesite,  but  intermingled  with  silicate  of  magnesia. 
No  fossils  were  found  in  this  white  rock,  and  it  was  considered  by 
Mr.  Tenison-Woods  to  be  a  decomposed  volcanic  ash  of  Miocene  age. 
There  is  a  very  fair  correspondence  in  general  character  between  the 
Eanny  Bay  Cliff  specimen  and  the  rocks  described  by  Mr.  Tenison- 
Woods,  and  it  remains  to  be  seen  whether  the  beds  described  as 
consisting  of  silicate  of  magnesia  may  not  really  be  of  radiolarian 
origin. 
In  spite  of  the  generally  imperfect  state  of  preservation  of  the 
radiolaria  in  this  white  rock,  the  generic  characters  can  in  many 
cases  be  ascertained,  and  in  some  the  species  as  well.  As  shown  in 
the  description  given  below,  the  three  sub-orders  of  the  Prunoidea, 
Discoidea,  and  Cyrtoidea  are  represented ;  but  the  first  named  is 
only  indicated  by  a  single  species  of  Cenellipsis.  Of  the  Discoidea, 
there  are  the  following  genera : — Astropliacus ,  2  sp.,  Lithocyclia,  1  sp., 
Amphibrachium ,  4  sp.,  Spongodiscus ,  3  sp.,  and  Spongolena,  1  sp.  ; 
while  the  following  are  the  genera  included  in  the  Cyrtoidea  : — 
Dictyomitra,  2  sp.,  Lithocampe ,  1  sp.,  and  Stichocapsa,  2  sp.  One 
peculiar  feature  in  this  assemblage  is  the  general  absence  of  forms 
with  freely-projecting  spines ;  in  only  three  instances  are  there 
short  secondary  spines  round  the  margins  of  discoidal  forms.  Erom 
1  ‘  Deport  on  the  Geology  and  Mineralogy  of  the  Northern  Territory/ 
No.  122, 1886,  South  Australia. 
Silica  . 
Iron  . . 
Alumina  . 
Lime  . 
Magnesia  . 
Soda  . 
Potash  . 
Loss  on  ignition 
