186  PROF.  J.  W.  JUDD  ON  INCLUSIONS  OF  TERTIARY  [May  1 893, 
granite  is  a  green  pyroxene.  As  a  rule,  however,  few  traces  of 
this  mineral  remain  in  the  fused  product,  it  having  lor  the  most 
part  been  broken  up  into  various  secondary  minerals,  among  which 
magnetite  is  usually  very  prominent.  The  magnetite  thus  pro¬ 
duced  can  always  be  distinguished  from  the  original  magnetite  and 
titanoferrite  of  the  rock ;  the  secondary  mineral  tends  to  form 
branched  and  radiated  filaments,  and  not  unfrequently  gives  rise  to 
the  extremely  delicate  trichites  which,  as  we  shall  see  hereafter,  are 
so  conspicuous  in  many  of  the  spherulitic  growths.  The  change  is 
very  similar  to  that  produced  in  the  hornblendes  of  the  Corriegills 
pitchstone  by  the  long  heating  of  that  rock  in  a  glass  furnace,  as 
shown  by  Mr.  Rutley.  Backstrdm,  in  the  work  already  quoted, 
has  pointed  out  that  the  ferro-magnesian  minerals  of  inclusions  of 
granite  in  basic  rocks  are  less  capable  of  resisting  the  action  of  the 
molten  magma  around  them  than  are  the  felspar-fragments,  and 
that  they  are  often  entirely  dissolved,  leaving  scarcely  a  trace  of 
their  former  existence.  The  cores  left  by  the  removal  of  these 
minerals  are,  he  says,  sometimes  surrounded  by  magnetite,  and 
possibly  also  by  secondary  quartz  and  biotite. 
The  original  magnetite  and  titanoferrite  are  apparently  unaltered, 
except  that  the  latter  mineral  sometimes  shows  the  beginnings  of 
change  into  sphene  (‘  leucoxene J),  and  the  crystals  are  scattered 
through  the  vitrified  mass.  Some  of  the  magnetite  may,  however, 
be  acted  upon  to  produce  the  pyrite  which  is  in  certain  cases 
tolerably  abundant. 
The  condition  of  the  minerals  in  these  masses  of  curiously 
altered  rock  will  thus  be  seen  to  be  strikingly  different  from  that 
of  the  former  minerals  in  the  granite  and  its  apophyses,  and  the 
changes  they  have  undergone  are  precisely  those  which  have  been 
described  as  occurring  in  enclosures  of  acid  in  basic  rocks  by 
Mitscherlich,  Lehmann,  Backstrdm,  Sauer,  and  other  authors. 
6.  The  Spherulitic  Growths  developed  in  the  Inclusions. 
If  we  now  turn  our  attention  from  the  porphyritic  crystals  to 
the  base  or  groundmass,  we  shall  find  in  the  fusion  of  the  micro¬ 
pegmatite  and  the  development  of  spherulites  in  the  glass  resulting 
from  this  fusion  an  exact  parallel  to  the  changes  which  have  been 
produced  in  plutonic  acid  rocks  by  the  action  of  heat,  as  described 
by  Sauer,  Bonney,  Backstrdm,  and  others. 
In  studying  the  remarkable  spherulitic  growths  which  have  been 
formed  in  the  glassy  mass  produced  by  the  fusion  of  the  micro- 
pegmatitic  groundmass  of  the  acid  rock  I  have  been  greatly  aided 
by  the  valuable  memoirs  of  Mr.  Iddings1  and  Mr.  Whitman  Cross/ 
1  J.  P.  Iddings,  ‘  Obsidian  Cliff,  Yellowstone  National  Park,’  Washington, 
1888,  issued  with  the  Seventh  Annual  Report  of  the  U.S.  Geological  Survey 
for  the  years  1885-6  ;  ‘  Spherulitic  Crystallization,’  Bull.  Phil.  Soc.  Washing¬ 
ton,  vol.  xi.  (1891)  pp.  445-464. 
Whitman  Cross,  ‘  Constitution  and  Origin  of  Spherulites  in  Acid  Eruptive 
Rocks,’  Bull.  Phil.  Soc.  Washington,  vol.  xi.  (1891)  pp.  411-444;  see  also 
F.  Rut  ley,  ‘  On  a  Spherulitic  and  Perlitic  Obsidian  from  Pilas,  Jalisco,  Mexico,’ 
Quart.  Journ.  Geol.  Soc.  vol.  xlvii.  (1891)  pp.  530-536. 
