418  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    1903.  [bull. 225. 
amphibolite  occur  in  the  Mariposa  slates.  These  amphibolites  are 
described  as  being  derived  from  diabase  or  gabbro.  The}'  are  in  part 
altered  to  serpentine. 
Previous  work  on  the  slate  deposits. — The  Placerville  folio,  No.  3, 
United  States  Geological  Survey,  published  in  1894,  contains  the 
results  of  detailed  geologic  work  by  Lindgren  and  Turner  in  the  area 
in  which  the  roofing-slate  deposits  occur.  At  that  date  the  roofing- 
slate  industry  had  not  assumed  its  present  importance,  though  all  the 
quarries  now  in  existence  had  then  been  opened.  The  existence  of 
roofing-slate  deposits  is  noted  in  the  text  of  the  folio,  and  the  locations 
of  the  quarries  are  indicated  on  the  map  showing  the  economic  geology 
of  the  area.  No  reference  is  made  to  the  "  green  slates,"  or  to  the 
dikes  cutting  the  Eureka  quarry. 
Excellent,  though  brief,  descriptions  of  the  different  quarries  and 
of  the  condition  of  the  slate  industry  at  various  dates  are  to  be  found 
in  the  reports  of  the  State  mineralogist  of  California,  particularly  in 
the  eighth  and  twelfth  reports. 
At  present  the  most  important  quarry  is  that  of  the  Eureka  Slate 
Company,  and  this  is  now  being  worked  on  a  large  scale.  This  quarry 
is  located  at  Slatington,  about  one-half  mile  southwest  from  Kelsey. 
Structural  relations  in  Eureka  quarry. — The  cleavage  planes  of  the 
slates  in  the  Eureka  quarry  strike  N.  25°  W.  The  dip  of  the  cleavage 
is  practically  vertical,  with  slight  local  variations  to  80°  E.  or  80°  W. 
The  upper  weathered  beds  in  the  quarry  are  overturned,  by  local 
pressure,  so  as  to  give  dips  of  40°  to  60°  to  the  east  or  west,  according 
to  local  conditions.  This  overturning  is  evidently  due  merely  to  the 
weight  of  the  overlying  soil  and  decomposed  slate,  and  the  effects  are 
shown  only  for  a  depth  of  from  3  to  15  feet.  It  is  of  interest,  how- 
ever, as  a  warning  against  accepting  dip  readings  taken  from  surface 
beds  of  the  slate. 
The  slate  bod}'  shows  rather  frequent,  but  narrow,  "  ribbons." 
These  ribbons  are  bands  (from  one-sixteenth  to  one-half  inch  thick 
usually,  but  occasionally  as  thick  as  2  inches)  of  material  differing  in 
composition  from  the  mass  of  the  slate.  They  are  in  general  more 
siliceous  than  the  normal  slate,  and  do  not  furnish  merchantable 
material.  Their  geologic  interest  arises  from  the  fact  that  they 
represent  differences  of  original  sedimentation.  The  plane  of  the 
ribbons  in  a  slate  quarry  is,  therefore,  the  plane  of  original  bedding. 
In  the  Eureka  quarry,  and,  indeed,  throughout  the  roofing-slate  belt, 
the  plane  of  original  bedding  seems  to  be  usually  within  10°  of  the 
plane  of  slaty  cleavage. 
The  slate  mass  is  cut  by  a  series  of  joints  parallel  to  the  "grain" 
of  the  slates,  striking  N.  55°  E.  and  dipping  from  70°  to  80°  N  W.  Joints 
across  the  " grain"  of  the  slate,  which  would  be  practically  horizontal, 
