740  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
nine-tenths  of  the  eruptives  of  the  area.  The  porphyry  appears  to  be 
the  surface  facies  of  the  granite,  and  seems  to  graduate  downward 
into  the  latter.  This  is  shown  where  erosion  has  been  great  and  has 
left  high  granite  hills,  which  are  often  capped  by  porphyry. 
Cambrian  rocks  directly  overlie  the  Archean  rocks,  with  uncon- 
formable relations. 
Keyes,70  in  1896,  considers  the  granites  and  porphyries  in  the  east- 
ern part  of  the  Ozarks.  Agreeing  with  Ha  worth,  he  finds  the  granites 
and  porphyries  to  be  different  facies  of  the  same  magma.  Further 
agreeing  with  Ha  worth,  he  finds  that  the  granite  occupies  the  lower 
ground,  the  porphyries  the  higher  ground,  and  that,  where  there  are 
gradations  between  the  two,  the  granites  are  at  the  base  of  the  hills 
while  the  porphyries  are  at  the  top,  with  transition  zones  between. 
The  granites  occupy  a  comparatively  small  area  in  the  northeastern 
part  of  the  district.  This  is  an  area  of  low  elevation  and  near  Mis- 
sissippi Kiver,  and  distribution  is  explained  as  due  to  differential 
erosion.  The  physiography  of  the  district  is  discussed,  and  the  con- 
clusion is  reached  that  the  crystalline  rocks  have  undergone  very 
considerable  erosion  since  Cambrian  time.  Agreeing  with  Van  Hise, 
it  is  held  as  probable  that  the  granites  and  porphyries  are  of  Al- 
gonkian  age.  A  deep  boring  near  Kansas  City  at  a  depth  of  2,500 
feet  penetrated  black  foliated  mica  schist,  which  has  the  characteristics 
of  the  Archean  rocks. 
Keyes,71  in  1896,  in  connection  with  a  description  of  the  clay  de- 
posits of  Missouri  by  Wheeler,  briefly  discusses  their  geological  oc- 
currence. Most  of  the  ore-bearing  conglomerates  of  Pilot  Knob  and 
vicinity,  heretofore  called  Algonkian,  are  believed  to  be  Cambrian. 
The  granites  and  quartz  porphyries  of  the  region  are  not  realty  of 
Archean  age,  as  generally  considered,  but  are  probably  Algonkian. 
In  chemical,  mineralogical,  and  structural  characters,  and  in  absence 
of  dynamic  effects,  they  differ  from  the  gneissic  and  schistose  rocks 
which  have  been  reached  in  deep  drill  holes,  and  therefore  they  are 
believed  to  be  younger  than  such  gneissic  and  schistose  rocks  (which, 
it  may  be  inferred,  are  believed  to  represent  the  Archean). 
The  geological  conditions  of  the  crystalline  rocks  are  unfavorable 
to  clay  deposits. 
Buckley  and  Buehler,72  in  1904,  map  and  describe  the  pre-Cam- 
brian  granites  of  southeastern  Missouri  in  connection  with  a  report 
on  the  building  stones  of  the  State. 
SUMMARY   OF  PRESENT  KNOWLEDGE. 
In  southeastern  Missouri  and  in  Camden  County  of  south-central 
Missouri  are  pre-Cambrian  islands  surrounded  by  Cambrian  sedi- 
ments.    The  pre-Cambrian  consists  mainly  of  granites,  felsites,  por- 
