776  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
Davis,26  in  1900,  in  a  general  account  of  a  trip  through  the  Colorado 
Canyon  district,  briefly  describes  certain  features  of  the  pre-Cambrian 
geology.  He  calls  attention  to  the  extraordinary  evenness  of  the  floor 
of  schists,  with  granite  dikes  (Archean),  upon  which  the  Chuar  and 
Unkar  terranes  (Algonkian)  rest.  The  floor  for  the  Paleozoic  strata 
is  somewhat  less  regular  than  the  floor  for  the  Unkar.  In  two  places 
the  pre-Cambrian  rocks  rise  higher  than  the  basal  Tonto  (Cam- 
brian) sandstone.  The  Archean  schists  beneath  the  Unkar  have  a 
steep  and  regular  slope,  indicating  uniform  resistance  to  erosion. 
Where,  beneath  the  Tonto,  they  show  a  bench,  it  is  taken  to  indicate  a 
softer  character  at  this  point,  probably  due  to  a  longer  period  of  pre- 
Tonto  weathering. 
Comstock,27  in  1900,  reviews  the  stratigraphy  of  Arizona. 
Blake,28  in  1901,  describes  the  salient  features  of  the  geology  of 
Arizona.  The  Santa  Catalina,  Eincon,  and  Rillito  mountains  con- 
sist largely  of  granitic  gneisses  and  schistose  rocks  of  pre-Cambrian 
age,  with  a  highly  complex  folded  structure,  and  exhibiting  a  high 
degree  of  metamorphism.  Taken  together,  these  mountains  may 
be  regarded  as  the  main  axis  of  ancient  uplift  and  of  insular  land 
areas  in  the  pre-Cambrian  and  Paleozoic  periods,  the  beginning  of 
the  "Arizona  Land." 
The  gneiss  of  the  southern  side  of  the  Santa  Catalina  near  Tucson 
is  regarded  as  Archean.  It  is  remarkable  for  its  regularity  of  strati- 
fication and  its  great  thickness,  probably  more  than  10,000  feet. 
It  occurs  in  great  tabular  masses  made  up  of  thin  layers,  which  when 
seen  laterally  give  the  appearance  of  evenly  stratified  shales  and 
sandstones.  In  the  same  range,  but  on  the  northeastern  side,  facing 
the  valley  of  the  San  Pedro,  another  formation  of  thinly  bedded  and 
highly  crumpled  mica  schist  in  sharply  defined  zigzag  folds  is  re- 
ferred to  the  Huronian,  and  is  given  the  name  Arizonan. 
Ransome,29  in  1904,  describes,  in  the  Pinal  Mountains  of  the  Globe 
district  of  Arizona,  mica  schists  with  occasional  bands  of  amphibole 
schists,  which  he  calls  the  Pinal  schists.  These  are  intruded  by 
quartz,  mica  diorite,  and  granite.  The  schists  and  intrusives  are 
unconformably  below  a  nonfossiliferous  series  supposedly  of  Cam- 
brian age.  The  schists  are  believed  to  represent  metamorphosed 
arkoses  or  grits.  They  are  probably  to  be  correlated  with  the  Vishnu 
series  of  the  Grand  Canyon,  provisionally  called  Algonkian  by  Wal- 
cott.  In  the  absence  of  other  criteria  the  Pinal  schists  are  referred 
to  the  pre-Cambrian. 
In  the  Mule  Mountains  of  the  Bisbee  district,  90  miles  to  the  south, 
are  similar  schists,  also  called  the  Pinal  schists.0  Evidence  of  sedi- 
mentary origin  is  less  satisfactory  than  in  the  Globe  district,  and 
a  The  Pinal  schists  probably  correspond  to  the  Arizonian  schists  of  Blake,  EDg.  and 
Min.  Jour.,  vol.   35,   1883,  pp.  238-239. 
