THE   CORDILLERAS.  821 
almost  horizontal  position  upon  the  upturned  edges  of  the  slates  and 
quartzites.  This  unconformity,  in  the  distance  at  which  it  may  be 
observed  and  in  the  masses  of  rocks  exposed,  is  remarkably  handsome. 
In  apparent  conformity  above  the  Devonian  at  Ouray  is  a  thick  Penn- 
sylvanian  Carboniferous  formation.  In  the  distance  of  about  5  miles 
in  which  this  quartzite  series  is  exposed  a  slate  band  is  found  five 
times.  In  going  north  the  dips  are  first  south  and  then  change  to  the 
north,  in  which  position  they  continue  until  the  Carboniferous  ap- 
pears. All  this  suggests  that  we  have  here  to  deal  with  a  folded  series, 
and  not  one  necessarily  of  very  great  thickness,  although  probably 
several  thousand  feet  thick.  As  Ouray  is  only  a  few  miles  from  Sil- 
verton,  the  argument  of  analogy  makes  it  probable  that  the  similar 
plainly  fragmental  slates  and  quartzites  south  of  Ouray  are  the 
equivalent  of  the  quartzites  of  Elk  Park.  The  facts  bear  against  the 
probability  of  a  transition  from  the  Devonian  into  the  quartzitic 
series  of  the  latter  place.  The  one  occurrence  in  which  this  transition 
is  definitely  asserted  is  perhaps  a  case  of  a  recomposed  rock  resting 
upon  a  crystalline  one.    Similar  occurrences  have  often  been  described. 
Emmons  (S.  F.),cg  in  1890,  states  that  on  the  north  slope  of  the  San 
Juan  Mountains,  near  Ouray,  are  more  than  10,000  feet  of  closely 
folded  quartzites,  conglomerates,  and  slates  of  pre-Cambrian  age,  and 
it  is  believed  that  the  quartzite  peaks  in  the  southern  portion  of  this 
region  are  probably  composed  of  the  same  series  of  rocks.  These  are 
referred  to  the  Algonkian. 
Cross,101  in  1899,  maps  and  describes  the  geology  of  the  Telluride 
quadrangle,  Colorado,  and  briefly  sketches  the  geology  of  the  San 
Juan  region,  of  which  the  Telluride  quadrangle  is  a  part. 
Along  Canyon  Creek  north  of  Stony  Mountain  is  a  small  body  of 
upturned  quartzites  with  an  intercalated  rhyolite  sheet,  which  have 
been  referred  to  the  Algonkian.  The  quartzites  are  coarse  and  grade 
into  fine  conglomerate. 
Cross  and  Spencer,102  in  1900,  describe  Algonkian  rocks  occurring 
in  the  center  of  the  Rico  Mountains  of  Colorado.  They  consist  of 
quartzites,  quartzitic  schists,  and  biotite  and  actinolite  schists.  The 
exposed  thickness  of  the  quartzites  is  more  than  350  feet,  and  prob- 
ably as  much  as  500  feet.  The  relations  of  the  quartzites  to  the 
schists  have  not  been  ascertained.  The  schists  and  quartzites  of  this 
area  are  similar  in  every  way  to  the  series  of  rocks  exposed  in  the 
upper  part  of  Animas  Canyon  and  in  adjacent  portions  of  the  Quartz- 
ite or  Needle  Mountains,  where  they  have  been  referred  to  the  Algon- 
kian by  Emmons  and  Van  Ilise.  The  quartzite  of  the  Rico  Moun- 
tains is  directly  along  the  strike  of  the  great  quartzite  belt  in  the 
Animas  Canyon  and  Needle  Mountains  area. 
Howe,103  in  1904,  describes  green  schists  in  the  pre-Cambrian  of 
the  Needle  Mountains,  in  San  Juan  and  La  Plata  counties,  southwest- 
