5G 
CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bdl^.  260, 
where  a  simple  fissure  in  impervious  rocks  gives  rise  to  large  lateral 
shoots  on  passing  through  beds  of  limestone. 
The  Bright  Diamond  and  Mineral  Farm  mines  are  instances  of 
immense  flat  bodies  of  ore  where  the  fissures  are  so  small  as  to  be 
often  difficult  to  detect. 
The  accompanying  section  of  Gold  Hill  (fig.  1),  very  much 
generalized  on  account  of  the  thinness  of  the  beds,  will  give  a  some- 
what inadequate  idea  of  the  varied  rocks  through  which  the  fissures 
pass  and  will  also  serve  to  illustrate  roughly  the  horizons  in  which 
a  lateral  extension  of  the  veins  most  frequently  takes  place. 
The  influence  of  the  wall  rock  on  the  form  of  the  ore  deposit  may 
be  further  understood  by  the  detailed  diagrammatic  drawings  (figs. 
2  and  3,  pp.  58,59). 
Horizontal  and  vertical  scale  the 
Quartz  it  e-monzonite-porphyryjj 
Black  shales      JH 
American-Nettie  quartzite 
Shales  and  quartzite 
Greenish  altered  shales  and  shaly  sandstones 
8right  Diamond  limestone 
Bright  Diamond  quartzite  - 
Reddish  and  greenish  shalesand  sandstones 
Sandy  shales  and  clay  shales 
with  thin  limestone  beds 
„     Quartzite 
Clay  shales, 
Quartzitic  sandstone 
Sandy  shales  and  sandstone 
Slack  Gir!  and  Pony-Express  limestone 
Massive.gray  or  white  cross  bedde 
sandstone 
Gold  Hill  fcongl 
Red  sandstone 
Mancos 
Dakota 
McElmo 
La  Plata 
Dolores 
Cutler 
Fig.  1. — Geological  section  of  Gold  Hill. 
The  ore  deposits  will  be  briefly  considered  in  the  following  order: 
1.  Fissure  veins: 
Silver-bearing  veins. 
Gold-bearing  veins. 
2.  Replacement  deposits  in  quartzite. 
3.  Replacement  deposits  in  limestone. 
FISSURE  VEINS. 
The  fissure  veins  are  readily  divisible  into  two  distinct  groups, 
(1)  silver-bearing  veins  and  (2)  gold-bearing  veins. 
SILVER-BEARING  VEINS. 
Location  and  general  feature*. — The  silver-bearing  veins  are  de- 
veloped largely  in  Dexter  Creek,  where  the  chief  interest  has  centered 
about  the  Bachelor,  Wedge,  and  Calliope  mines,  and  on  the  east  side 
