304  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
In  the  West  Humboldt  range  a  variety  of  crystalline  schists  are 
found  unconformably  upon  the  granite,  there  being  no  tendency  for  a 
passage  to  occur  between  the  two  rocks  and  dikes  of  granitic  material 
invading  the  schists.  In  the  schists  are  roundish  areas  of  quartz, 
which  might  be  explained  on  the  supposition  that  they  are  the  pebbles 
of  conglomerates,  but  they  are  more  probably  an  aggregation  formed 
during  metamorphism. 
In  the  Montezuma  range  the  granite  of  Trinity  peak  is  undoubtedly 
of  eruptive  origin,  as  may  be  determined  from  its  general  habitus  and 
from  its  penetrating  the  Archean  schists  in  well  denned  dikes. 
In  the  Pah-tson  range  the  Archean  nucleus  consists  of  crystalline 
schists,  a  limited  amount  of  granite,  and  a  subsequent  granite  which 
has  cut  through  the  older  granites  and  schists.  These  are  all  cut  by 
dikes  of  later  age,  but  supposed  to  be  Archean. 
The  Truckee  range  is  composed  of  schists  and  granite  representing 
two  periods  of  formation. 
The  Pea  Vine  mountains  consist  of  a  series  of  conformable,  highly 
altered  beds,  striking  from  north  50°  to  65°  east,  made  up  for  the  most 
part  of  fine  grained  quartzite  strata,  riven  in  every  direction  with  mi- 
nute fissures,  which  are  filled  with  ferruginous  material. 
It  is  remarked  that  in  the  absence  of  any  granitic  dikes  penetrating 
the  stratified  series,  or  of  peculiar  local  metamorphism,  or  general  evi- 
dence of  intrusion,  the  bodies  are  usually  referred  to  the  Archean. 
Only  in  cases  where  the  granite  is  actually  seen  to  penetrate  the  open- 
ings in  the  strata  is  it  safe  to  refer  it  to  a  later  age  than  the  sedimen- 
tary series. 
In  the  analytical  map  the  rocks  are  divided  into  two  classes,  the  inten- 
tion being  to  discriminate  those  formations  which  are  sedimentary  from 
the  class  of  eruptive  rocks ;  but  this  line  can  not  be  drawn  with  pre- 
cision, because  the  series  of  gneisses  pass  into  the  massive  layers  and 
because  limited  bodies  of  granite  which  are  massive  might  if  more 
largely  exposed  pass  into  crystalline  schists  or  other  Archean  sedi- 
mentary rocks. 
It  is  not  easy  to  analyze  those  subtle  appearances  which  lead  the 
observer  to  incline  to  one  or  the  other  of  the  two  possible  modes  of  ori- 
gin of  a  granite  outcrop.  Parallelism  of  bedding,  and  even  parallelism 
of  the  arrangement  of  minerals,  are  consistent  with  the  theory  of  an 
eruptive  origin.  Certain  masses  of  gneissoid  granite  appearing  in 
the  great  eruptive  granite  body  of  the  Sierra  Nevada  show  quite  as 
much  parallelism  of  bedding  and  internal  arrangement  of  minerals  as 
the  Eocky  mountain  granites  to  which  we  have  assigned  a  metamor- 
phic  origin ;  yet  the  Sierra  field,  as  a  whole,  is  clearly  eruptive.  But 
at  the  same  time,  in  the  intimate  arrangement  of  the  mineral  particles 
and  in  the  mode  of  contact  between  the  various  mineral  ingredients, 
there  is  a  certain  broad  uniformity  in  all  the  eruptive  granites  which 
produces  a  characteristic  impression  upon  the  eye*    On  the  contrary, 
