SPHRR    AND 
OAKKKY. 
GEORGETOWN    MTNTNG    DISTRICT,    COLORADO.  101 
jgxtreme  western  part  of  the  county,  has  an  elevation  of  14,841  feet,a 
while  Mount  Evans,  8  miles  south  of  Georgetown,  is  14,260  feet b 
above  sea  level.  From  a  point  where  the  deep,  narrow  valleys  form 
a  conspicuous  part  of  the  scene  the  mountains  appear  extremely  steep 
and  rugged.  From  a  higher  point,  however,  the  aspect  is  that  of  an 
old,  rolling,  hilly  plateau,  deeply  trenched  by  valleys  which  have  not 
usually  worn  back  far  enough  to  cause  sharp  intervening  ridges — 
that  is,  a  considerable  portion  of  the  upland  has  been  left  undissected. 
These  trenched  valleys  are  chiefly  pre-Glacial;  their  upper  portions, 
however,  have  often  been  occupied  by  glaciers,  which  have  left  lat- 
eral, terminal,  and  ground  moraines.  Near  the  base  of  steep  slopes 
there  is  often  a  heavy  coating  of  talus. 
A  topographic  survey  of  the  Georgetown  and  Central  City  quad- 
rangles, on  the  scale  of  1  mile  to  the  inch,  was  made  by  Mr.  Frank 
Tweedy  during  the  summer  of  1903.  The  maps  are  now  being 
[engraved.  A  special  topographic  map  of  the  Silver  Plume  mining 
area,  on  the  scale  of  1,000  feet  to  the  inch,  was  made  by  Mr.  Pearson 
Chapman  during  the  summer  of  1904. 
GENERAL  GEOLOGY. 
The  rocks  of  the  Georgetown  district  belong  almost  entirely  to  the 
oldest,  presumably  pre-Cambrian,  formations  of  the  Rocky  Moun- 
tains. Later  than  these  are  intrusive  bodies  of  porphyry,  and  the  sur- 
ficial  deposits  of  glacial  and  fluviatile  origin.  The  pre-Cambrian 
formations  which  comprise  the  main  mass  of  all  these  mountains 
include  gneisses  and  schists,  granites,  pegmatites,  and  basic  igneous 
rocks  resembling  diorites  or  metagabbros.c 
The  basal  gneisses. — Certain  gneisses  and  schists  are  the  oldest 
jocks  of  the  region.     The  commonest  of  these  is  a  black  biotite-gneiss 
r  schist  with  varying  proportions  of  biotite,  feldspar,  and  quartz  as 
the  chief  constituents.     There  are  also  limited  areas  where  a  horn- 
)l(Mide-gneiss  of  apparently  contemporaneous  origin  is  found.     The 
)lder  gneisses  in  certain  places  are  much  metamorphosed  and  crum- 
bled.    Over  large   areas,   moreover,  the  gneiss  is  very   thoroughly 
utruded  by  varying  amounts  of  an  acidic  rock  which  usually  has  the 
brm  of  pegmatite,  but  occasionally  shows  that  of  granular  alaskite 
nd  more  rarely  that  of  an  acidic  granite.     These  intrusives  often 
fecur  in  tongues  and  lenses,  following  the  contorted  laminae  of  the 
neiss  in  such  a  way  that  both  the  intrusive  and  the  gneiss  appear  to 
ave  been  folded  together.    The  connection  between  pegmatitic  dikes 
utting  across  the  schists  and  the  interlaminated  pegmatitic  layers  is. 
n 
U.  S.  Geol.  Survey  Terr.  [Ilayden]. 
Topographic  map  of  the  Georgetown  quadrangle,  by  Mr.  Frank  Tweedy. 
Microscopic  work  not  yet  undertaken. 
