122  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1900,  PART    I. 
rock.  Chromite  and  magnetite  are  in  general  present  as  dissemH 
nated  grains,  though  in  places  within  the  peridotite  area  there  are 
segregations  of  almost  pure  chromite.  The  peridotite  readily  breaks 
down  to  a  dark-greenish  serpentine,  a  rock  that  in  the  Nickel  Moun- 
tain region  is  widely  distributed  as  small  isolated  patches  and  elon- 
gated masses,  the  trend  of  which  is  northeast  and  southwest.  Such 
an  elongated  mass  of  serpentine  extends  for  several  miles  both  to  the 
northeast  and  to  the  southwest  of  Nickel  Mountain.  In  some  places 
the  band  is  narrow;  in  others  it  is  more  than  a  mile  in  width.  The 
serpentine  has  but  a  thin  covering  of  soil,  winch  is  comparatively  free 
from  vegetation. 
Other  igneous  rocks  in  this  region  are  less  basic  than  the  peridotite 
and  may  be  designated  as  greenstones  and  dacite  porphyries.  The 
greenstones  comprise  several  types  of  rock,  all  of  which  are  more 
or  less  dull  green  in  color.  They  vary  in  texture  from  fine-grained 
and  compact  to  coarsely  granular.  Most  of  them  are  considerably 
altered,  but  where  fresh  are  usually  found  to  consist  essentially  of 
pyroxene  and  soda-lime  feldspars.  The  dacite  porphyries  are  rather 
fine-grained,  light-colored  rocks  and  are  much  less  abundant  than 
the  serpentines  and  greenstones.  The  chief  minerals  present  arc  ! 
quartz  and  soda-lime  feldspars,  both  of  which  in  many  places  form 
distinct    phenocrysts.     Ferromagnesian   minerals  are  subordinate. 
The  peridotite  appears  to  have  cut  up  through  the  greenstones, 
but  was  itself  intruded  by  the  dacite  porphyries. 
The   sedimentary    rocks   are    Meso/oic   and   Tertiary  in   age.     The 
systems    represented    are    Jurassic;    Cretaceous,    and    Eocene.     The -J 
Jurassic  consists  chiefly   of  sandstones  and  subordinate^    of  shales,..! 
conglomerates,  and  cherts.     The  rocks  as  a  rule  show  veining  and 
pronounced    lithification.     Fossils    are    scarce,    but    the    distinctly  | 
Jurassic   form   Aucella  erringtoni  has  been  found.     The  Cretaceous 
rocks  consisl   chiefly  of  conglomerates,  sandstones,  and  shales,  and 
their  Io>h1>  correspond  to  those  of  the  Knoxville  and  Ilorsetown  of 
California.     There  is  evidence  of  a  slight  unconformity  between  the  I 
beds  representing  these  two  formations.     The  Knoxville  rocks  are  J 
lithified,  but   for  the  most  part  not  nearly  so  strongly  as  the  rocks 
of  the  Jurassic.     The  Eocene  deposits  consists  of  yellowish  sandstone, 
shale,  and  conglomerate,   the  stratification  of  the  rocks  being  well  i 
preserved. 
A  great  unconformity  can  be  traced  between  the  Jurassic  and  the 
Cretaceous,  and  a  somewhat  less  important  unconformity  separates 
the  Cretaceous  and  the  Eocene. 
All  the  igneous  rocks  of  the  region  are  younger  than  the  Jurassic, 
some  are  younger  than  the  Cretaceous,  and  all  are  older  than  the 
Eocene. 
