306  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull. 86.  j 
SUMMARY   OF   RESULTS. 
It  is  evident  that  west  of  the  Wasatch  no  such  detailed  and  careful 
work  was  done  by  the  Fortieth  Parallel  Survey  as  in  that  range  and  in 
the  ranges  to  the  east.     In  the  Nevada  ranges,  which  contain  granite 
only,  the  reference  of  the  rocks  to  the  Archean  or  to  a  later  period  is 
based  upon  too  little  evidence,  and  it  can  only  certainly  be  said  that  in 
each  case  the  granites  are  older  than  the  oldest  sedimentary  rocks  with 
which  they  are  in  contact  and  do  not  cut.     Some  of  the  granite  areas 
are  said  to  be  as  late  as  the  Jurassic,  as  for  instance  that  of  the  W ashoe 
range,  which  is  in  eastern  Nevada.    That  this  is  an  intrusive  could  be 
determined,  because  it  cuts  the  pre- Jurassic  rocks ;  but  in  the  numerous 
cases  in  western  Nevada  the  undisturbed  sedimentary  rocks  adjacent^ 
to  the  granite  ranges  are  Triassic  or  Jurassic;  so  that  granites  earlier : 
than  these  periods,  but  far  later  than  the  Archean,  would  show  no  3 
structural  evidence  of  their  late  age. 
It  is  now  well  settled  that  granitic-textured  rocks  vary  into  porphy- j 
ritic  forms,  but  to  the  Fortieth  Parallel  Survey  belongs  the  credit  of  anj 
early  recognition  of  this.  In  the  Gosi-Ute  and  Franklin  butte  ranges,] 
in  which  the  rocks  are  referred  to  the  Archean,  the  granites  are  said  to] 
grade  into  granite-porphyries,  and  at  Franklin  butte  into  a  genuine^ 
felsite-porphyry. 
In  the  Gosi-Ute  the  granite-porphyry  is  so  associated  with  crystalline^ 
limestone,  referred  to  the  Archean,  as  to  lead  to  the  conclusion  that^ 
both  the  granite  and  granite-porphyry  are  metamorphosed  sedimentary  j 
rocks.    It  seems  far  more  probable  that  they  are  eruptives  later  than  the 
crystalline  limestone  and  have  been  the  cause  of  its  metamorphism. 
This  explanation  was  applied  by  the  Fortieth  Parallel  surveyors  to  the] 
case  of  the  interstratiiication  of  marbles  and  granitic  porphyry  in  the] 
hills  between  the  Antelope  and  Schell  creek  ranges.    In  the  western  hall 
of  Nevada,  especially,  are  abundantly  found  late  rhyolites,  trachytes,-] 
porphyries,  etc.,  so  that  it  may  be  said  that  this  has  been  a  region  of 
great  volcanic  activity  until  late  time,  the  chemical  composition  ofl 
many  of  the  rocks  being  the  same  as  granite.     While  probably  mistakes 
have  been  made  as  to  the  age  of  the  granites  in  individual  cases,  thej 
general  point  which  King  makes   that  the  ancient  granites  have  a| 
crushed,  irregular,  and  confused  mode  of  arrangement  of  the  minerals  is? 
one  of  considerable  weight,  although  it  would  have  no  bearing  upon 
their  origin ;  for  this  condition  of  the  minerals  would  by  many  geolo- 1 
gists  be  taken  as  merely  evidence  of  powerful  dynamic  action  which 
has  produced  the  present  confused  condition  in  an  eruptive  granite, 
rather  than  the  result  of  metamorphic  processes  upon  a  sedimentary- 
rock.    The  exhibition  of  these  characters  by  one  mass  of  granite  and 
their  lack  in  an  adjacent  one,  indicates  that  the  former  is  of  greater 
age  because  the  other  has  escaped  the  effects  of  dynamic  action. 
In  the  cases  of  the  Schell  creek,  Egan,  Pogonip,  and  Piiion  ranges, 
where  below  the  Olenellus  Cambrian  there  is  a  great  thickness  of  in 
