102  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
however,  often  perfectly  displayed  in  a  single  outcrop,  and  there 
seems  to  have  been  little  visible  folding  or  contortion  subsequent  to 
this  injection.  The  layers  of  pegmatite  have  been  injected  between 
the  laminae  of  the  schist  in  all  proportions,  so  that  sometimes  the 
original  black  schist  or  gneiss  greatly  predominates;  sometimes  half 
gneiss  and  half  pegmatite  is  present;  again  the  pegmatite  predomi- 
nates over  the  gneiss,  and  in  other  places  simply  shreds  or  breccia-like 
fragments  of  gneiss  may  be  found  as  inclusions  in  almost  pure  peg- 
matite. 
Where  the  pegmatite  is  very  abundant  the  intervening  gneiss  i; 
often  metamorphosed  to  a  gray  granitic  gneiss,  evidently  the  resul  i 
of  saturation  with  the  alaskitic  fluid.     This  mingling  with  the  orig 
inal  gneiss  and  recrystallizing  has  apparently  sometimes  produced  il 
granitic  rock  more  basic  than  the  injected  material. 
One  phase  of  granitic  or  quartzitic  gneiss,  which  contains  nodula 
or  pebble-like  masses  scattered  through  it,  occurs  rather  frequently 
These  nodules,  which  are  composed  chiefly  of  quartz,  with  some  silli 
manite,  vary  in  size  from  one-fourth  inch  to  over  3  inches  in  diameter! 
Their  general  arrangement  is  like  that  of  conglomerate  pebble:  i 
Further  study  may  justify  the  already  strong  opinion  that  this  pai 
ticular  gneiss  is  of  sedimentary  origin  and  is  a  metamorphosed  coi 
glomerate. 
The  gneisses  often  have  biotite  as  the  predominant  mineral,  wit 
clear  to  smoky  quartz  and  whitish  feldspar  in  about  equal  propo 
tions.  The  accessory  minerals  include  muscovite,  sillimanite,  maj 
netite,  and  garnets.  The  compact,  vitreous  gneiss  is  composed  almo 
entirely  of  quartz,  while  biotite,  feldspar,  and  muscovite,  as  well 
magnetite,  garnet,  and  sillimanite,  may  be  termed  accessory  mineral 
Diorites. — Basic,  igneous  hornblendic  rocks  of  dark  color,  resei 
bl ing  diorites,  occupy  comparatively  limited  patch-like  areas,  and 
a  few  cases  form  narrow  dikes.     These  rocks  are  very  likely  of  two 
more  ages. 
Outcrops  near  the  Illinois  and  Wisconsin  mines  were  noted  whe»j 
the  diorite  appears  to  be  sheared,  and  seamed  thoroughly  with  pe  | 
matite.     On  close  observation,  however,  the  masses  are  seen  to 
practically  unsheared  and  the  pegmatite  to  be  a  contemporanec 
segregation  product.     From  a  single  locality  may  be  obtained  exctj 
lent  specimens  which  show  all  the  phases  of  segregation  of  the  dior 
through  a  basic-growing  series  culminating  in  a  hornblendite  a 
through  an  acidic-growing  series  culminating  in  pegmatite  or  e^ 
in  pegmatitic  vein  quartz. 
A  very  basic  formation,  chiefly  biotite  with  a  small  amount  of  ho:  ] 
blende,  is  sometimes  found  in  dike-like  bodies.  This  rock  is  probabj 
a  sheared  and  metamorphosed  form  of  diorite. 
