420 
CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull.  225. 
Analyses  of  green  slates,  Eureka  quarry,  Slatington,  Cal. 
Silica  (SiOj) 
Alumina  (Al,<)3) 
[ron  oxides  (  FeO,  F203) 
Lime  (CaO) 
Magnesia  (MgO) 
Sulphur  (S) 
Alkalies  (K20,  Na,0).-. 
( larbon  dioxide  (C02) 
Water 
(1) 
(2) 
Per  cent. 
Per  cent. 
45.15 
47.30 
16.  33 
15.  53 
8.42 
8.00 
6.42 
7.83 
8.72 
7.86 
(") 
.12 
(«) 
3.17 
|  11.28 
9.92 
Average. 
Per  cent. 
46.  22 
15.  93 
8.21 
7.  12 
8.  29 
.12 
3.17 
10.60 
«Not  determined. 
These  analyses  have  been  compared"  with  a  series  of  36  analyses  of 
American  roofing  slates  derived  from  clays  by  pressure,  and  remark- 
able differences  in  composition  are  apparent.  The  green  slate,  on  the 
other  hand,  approximates  closely  in  composition  to  certain  "basic" 
igneous  rocks  of  the  district,  and  it  is  probable  that  it  was  derived 
from  a  gabbro  or  similar  rock. 
The  "green  slate"  is  in  reality  grayish  green  in  color.  It  splits 
readily,  though  not  with  as  smooth  a  surface  as  the  black  slate.  It  stands 
punching  and  trimming  well,  and  is  sufficiently  strong  for  roofing  use. 
Considering  its  origin  and  composition  it  is  probable  that  it  will  be  a 
highly  durable  slate,  holding  its  color  well.  At  present  it  is  sold 
entirely  for  trimming  and  lettering  on  black  slate  roofs,  for  which 
purpose  it  is  particularly  well  adapted,  giving  a  strong  but  pleasant 
color  contrast. 
Chili  Bar  Slate  Company  quarry. — This  quarry  is  located  about  3 
miles  north  of  Placerville,  in  sec.  36,  T.  11  N.,  R.  10  E.,on  the  south 
side  of  the  South  Fork  of  American  River,  a  few  hundred  yards  east 
of  the  Placerville-Kelsey  stage  road. 
This  is  the  oldest  quarry  in  the  district,  having  been  opened  about 
twenty  years  ago.     It  has  been  shut  down  since  1897. 
Several  openings  were  made  in  a  bluff  forming  the  river  bank  at  this 
point.  In  the  easternmost  of  these  openings,  which  is  about  10  feet 
high  and  30  feet  wide,  a  rather  poor  slate  with  irregular  joints  is 
shown.  The  cleavage  strikes  N.  20°  W.,  and  dips  75°  E.  The  west- 
ernmost opening  is  small,  with  a  tunnel  which  was  apparently  run  in 
on  a  band  of  better  slate.  The  slate  piled  in  the  yard  has  kept  its 
color  fairly  well. 
It  seems  possible  that  this  quariy  may  be  flooded  at  high  water. 
Both  it  and  the  one  next  mentioned  (San  Francisco  quarry)  are  badly 
a  Jour.  Geol.,  vol.  12,  1904,  p.  26. 
