vanhise.]  THE    CORDILLERAS.  323 
long-continued  one,  and  was  far  advanced  before  the  deposition  of 
the  fragraental  rocks.  If  it  should  be  held  that  this  granitic  area  is 
eruptive  as  a  whole  and  later  than  the  quartzite,  the  question  arises, 
Why  is  it  that  nowhere  do  any  of  the  rocks  which  belong  in  it  cut  the 
quartzites  ?  If,  on  the  other  hand,  it  is  maintained  that  it  is  nietainor- 
phic  in  origin,  the  question  arises,  Why  is  it  that  all  parts  of  it  have  be- 
come so  completely  crystalline  while  the  quartzite  is  still  so  near  its 
original  condition? 
LITERATURE   OF   THE  LA  PLATA  MOUNTAINS. 
Holmes,68  in  1877,  places  the  La  Plata  mountains  in  the  metamorphic 
belt  because  the  central  portion,  as  exposed  in  the  deep- cut  valley  of  the 
La  Plata  river,  is  composed  of  uplifted  and  altered  sedimentary  rocks ; 
but  there  are  associated  with  these  a  very  considerable  area  of  eruptive 
rocks  with  a  resulting  great  complication  of  structure.  This  metamor- 
phic group  seems  to  be  a  prolongation  to  that  to  the  northeastward 
about  the  Animas.  Against  attributing  any  great  amount  of  change 
in  the  sedimentary  rocks  to  the  presence  of  the  trachyte  is  the  fact  that 
in  the  neighboring  groups  of  mountains  of  trachytic  origin  there  is  lit- 
tle or  no  metamorphism  apparent.  In  the  central  part  of  the  altered 
area  the  metamorphic  mass  proper  extends  up  to  and  includes  the  red 
beds.  As  one  of  the  best  examples  of  the  metamorphism  may  be  men- 
tioned that  on  the  west  face  of  the  first  mountain  south  of  Hesperus. 
Here  a  mass  of  metamorphic  shales  abuts  or  is  welded  to  the  trachyte 
face  of  the  mountain.  The  exact  point  of  contact  can  not  be  determined, 
as  the  metamorphism  has  been  so  complete  that  the  shales  seem  to 
change  gradually  into  trachyte.  Away  from  the  trachyte  they  grad- 
ually assume  the  appearance  of  a  massive  grayish  yellow  quartzite,  and 
in  a  mile  or  more  from  the  place  of  contact  assume  a  shaly  character 
and  dark  color. 
Endlich,49  in  1878,  gives  a  general  discussion  of  the  formations  of 
Colorado.  In  preference  to  the  word  Azoic  the  word  Prozoic  is  used. 
Belonging  to  this  group,  in  southern  Colorado,  is  an  extensive  series 
comprising  gneisses,  granites,  various  schists  and  diorites.  Of  these 
the  first  named  appear  to  be  the  oldest,  as  may  be  inferred  from  the 
relations  to  the  granites  more  particularly.  The  schists  are  in  subor- 
dinate quantity.  It  often  is  a  matter  of  difficulty  to  discriminate  be- 
tween the  Prozoic  and  the  next  group  of  metamorphic  rocks.  This 
latter  is  the  most  varied  and  enormous  in  its  development.  Large 
areas  are  covered  by  rocks  of  this  group,  which  occur  in  almost  end- 
less variety.  In  several  instances  localities  may  be  observed  where  the 
transition  from  undoubted  sedimentary  into  metamorphic  beds  is  evi- 
dent. This,  however,  must  be  considered  as  an  exception  rather  than 
the  rule.  The  gneiss  is  the  oldest  of  the  metamorphic  rocks  in  the  dis- 
trict examined.  Micaceous,  hornblendic  and  chloritic  schists  occur  as 
such  associated  with  other  metamorphic  rocks.    Frequently  they  are 
