324  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
due  to  the  substitution  of  minerals  within  the  gneiss,  but  they  are  also 
found  totally  independent  thereof.  If  a  suggestion  may  be  offered 
which ,  however,  can  not  at  present  be  proved,  the  author  would  say 
that  argillaceous  sandstones  form  granite.  With  the  decrease  or  in- 
crease of  argillaceous  matter  in  the  sandstone  the  quantity  of  feldspar 
in  the  granite  stands  in  direct  proportion.  Siliceous  sandstones  form  ' 
quartzites.  Shales,  arenaceous  in  part,  are  changed  into  gneisses,  and 
if  the  quartz  in  them  is  predominant  they  turn  into  schists.  Quartzites 
can  not  be  generally  classed  with  the  metamorphic  rocks,  but  in  the 
Quartzite  mountains  a  complete  alteration  of  the  original  sandstone 
has  taken  place,  although  stratification  has  been  retained  in  a  measure. 
Granite  is  the  most  representative  species  of  the  metamorphic  group. 
It  is  younger  generally  than  the  schistose  rocks  occurring  with  or  near 
it.  In  the  Quartzite  mountains  there  is  a  direct  transition  from  sedi- 
mentary beds  into  typical  granite.  A  large  part  of  the  granites  in 
southern  Colorado  is  regarded  as  metamorphosed  Silurian,  Devonian, 
and  in  rare  instances  even  Carboniferous  strata. 
SUMMARY  OF   RESULTS. 
The  greater  parts  of  the  Front,  Wet,  Sawatch,  Park,  and  Quartzite 
ranges  and  the  crystalline  rocks  of  the  Gunnison  and  Grand  are  a 
completely  crystalline  complex  of  rocks  which  are  certainly  pre-Cam- 
brianj  for,  resting  upon  these  ranges  un conformably  and  bearing  de- 
bris from  the  older  series  are  the  fossiliferous  Cambrian.  These  con- 
tacts are  found  both  on  the  east  side  of  the  Front  range  and  in  the 
parks  along  the  Sawatch,  as  well  as  af  Eagle  river  in  the  Gunnison  and 
Grand  region.  The  granite-gneiss-schist  complex  of  the  Quartzite 
mountains  also  without  much  question  belongs  in  the  same  position. 
The  relations,  so  excellently  described  by  Marvine  and  Stevenson, 
between  the  nearly  structureless  granites  constituting  the  core  of  the 
ranges  and  the  well  laminated  schists  and  gneisses  are  those  between 
the  granites  and  associated  crystallines  described  in  Massachusetts  by 
the  elder  Hitchcock  in  1860,  by  King  along  the  fortieth  parallel,  by 
Lawson  about  the  Eainy  lake  and  the  lake  of  the  Woods,  and  by  Win- 
chell  in  northeastern  Minnesota.  That  is,  in  passing  from  a  schistose 
to  a  granitic  area  the  finely  laminated  schists  become  coarser  and 
coarser ;  then  appear  thin  belts  of  gneiss,  which  become  more  and  more 
prominent  until  the  rock  has  changed  to  a  gneiss,  and  this  by  imper- 
ceptible stages  passes  into  a  granitoid  gneiss,  then  into  a  granite.  A 
whole  or  a  part  only  of  the  laminated  rocks  may  be  cut  by  granite  veins, 
while  oftentimes  there  are  considerable  masses  of  granite  in  the  schists 
of  the  same  character  as  the  main  granite  mass,  the  contacts  being  ex- 
ceedingly sharp.  In  a  few  cases  in  the  massive  granites  are  found 
fragment-like  areas  of  the  schists.  These  imperfectly  summarized  re- 
lations are  regarded  by  Stevenson,  Marvine,  and  the  other  writers  to 
be  evidences  of  the  metamorphic  character  of  the  whole  series,  while 
