334  PRE-CAMBRIAN   ROCKS    OF   NORTH    AMERICA.  [bull.  86. 
are  used  planes  of  structure  or  lamination.  At  one  section  was  found 
granite,  upon  both  sides  of  which  is  white  limestone;  next  to  the  latter, 
on  one  side  is  quartz  rock,  which  is  followed  by  chlorite-slates.  If  the 
structural  relations  were  regarded  as  conclusive  evidence,  the  whole 
series  would  necessarily  be  considered  metamorphic;  but  there  is  little 
reason  to  doubt  that  the  granite  is  eruptive.  The  metamorphism  in 
the  limestone  is  complete  and  resembles  the  coarsely  crystalline  white 
limestones  of  Sussex  county,  New  Jersey.  There  is  no  indication  as 
to  the  age  of  the  limestone  or  quartz  rock,  but  there  is  some  reason  to 
regard  them  as  Carboniferous,  for  these  are  the  nearest  known  forma- 
tions of  limestone  which  are  recognizable  by  fossils.  On  the  section 
of  the  Canada  de  las  Uvas  the  rocks  are  similar  to  those  of  the  western 
slopes  of  the  Tejon.  Along  the  Mojave  river  the  rocks  consist  of  meta- 
morphic  slates,  very  compact  and  so  much  changed  as  to  resemble 
granite.  In  the  Colorado  desert  the  most  of  the  metamorphic  rocks 
are  highly  laminated  and  contain  lenticular  beds  of  limestone.  In  the 
gold  region  talcose  and  clay-slates  are  the  prevailing  rocks,  and  in  gen- 
eral present  a  low  degree  of  metamorphism.  Next  to  the  slate  in 
importance  is  white  crystalline  limestone. 
Emory,78  in  1857,  states  that  in  southern  California  there  is  a  great 
preponderance  of  crystalline  metamorphic  granite  pertaining  to  the 
older  Paleozoic  series  of  rocks  and  an  entire  absence  of  any  member 
of  the  lower  Paleozoic  or  secondary  rocks  in  their  regular  stratified 
character.  The  central  axes  are  represented  by  somewhat  variable 
granite,  assuming  in  some  places  a  close  syenitic  texture  and  at  other 
times  there  is  a  preponderance  of  mica.  Belonging  with  the  granitic 
series,  particularly  on  the  eastern  side  of  the  range,  are  mica  and  tal- 
cose slates. 
Newberry,79  in  1861,  describes  the  great  mass  of  the  Peninsular 
mountains  east  of  San  Diego  as  composed  of  granitic  and  gneissoid 
rocks3  which  are  similar  to  most  of  the  granites  of  the  other  systems 
of  the  Colorado;  that  is,  a  predominance  of  the  feldspathic  over  the 
hornblendic  ingredients.  Where  the  Colorado  cuts  through  the  Choco- 
late mountains  they  are  composed  of  gneisses  traversed  by  veins  of  gran- 
ite and  quartz.  The  gneissoid  rocks  are  frequently  foliated  and  much 
convoluted.  Their  aspect  is  such  as  to  lead  an  observer  more  readily  to 
refer  them  to  a  metamorphic  origin  than  any  other  rocks  seen  on  the 
route.  The  great  mass  of  Monument  mountains  is  a  coarse,  massive 
feldspathic  granite.  On  both  sides  of  the  granitic  axis  are  highly  meta- 
morphosed conglomerate  and  sandstone.  The  principal  mass  of  the 
Mojave  mountains  is  composed  of  white  granite,  traversed  by  numerous 
veins  of  quartz.  The  Black  mountains  as  a  whole  are  characterized  by 
the  prominence  of  eruptive  rocks,  such  as  massive  granite,  trap,  por-  ■ 
phyry  and  trachyte,  and  the  rarity  of  gneiss,  mica  slate,  clay-slate, 
etc.,  which  are  probably  metamorphic.  In  the  lower  Colorado  canyon, 
unconformably  below  the  Potsdam  sandstone,  is  granite,  which  is  cut 
