416  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    1903.  [bull.  225. 
pseudomorphic  rhombs,  0.08  millimeter  in  diameter,  of  ehlorite  scales 
were  found. 
Whether  the  two  extra  foliations  will  prove  detrimental  in  this 
particular  slate  could  be  determined  by  physical  tests  or  by  continued 
use;  but  secondary  cleavage  in  roofing  slate  generally  facilitates  frac- 
tures parallel  to  it,  and  is  therefore  regarded  as  a  very  doubtful 
feature.  In  this  slate  the  bedding  will  probably  operate  like  a  second 
ww  false  cleavage." 
6.  Light-green  slate  (phyllite)  from  an  unnamed  locality. — This 
slate  occurs  in  association  with  the  reddish  slate  (No.  1).  It  is  more 
greenish  than  No.  5,  being  a  delicate  pea  green.  Its  surface  is 
smoother  than  that  of  No.  5,  owing  to  the  absence  of  the  transverse 
foliation,  and  has  a  waxy  luster.  It  is  sonorous  and  splits  with  great 
facility  and  fineness.  The  aggregate  polarization  is  brilliant  and  the 
parallelism  of  the  sericite  is  almost  perfect.  A  microscopic  bed  of 
quartz  grains  with  chlorite  and  muscovite  lies  in  the  cleavage,  which  is, 
therefore,  the  bedding  plane  also.  The  "grain"  is  indicated  by  the 
transverse  position  of  some  of  the  muscovite  scales  and  prismatic 
crystals.  The  material  is  fine  grained.  Quartz  is  not  very  abundant, 
but  attains  a  diameter  of  0.037  millimeter.  Slate  needles  are  abundant 
end  are  as  much  as  0.012  millimeter  in  length.  Muscovite  and  chlorite 
scales  are  present,  and  to  the  latter  the  slate  owes  its  color.  Some 
opaque  granules  (limonite?)  and  pyrite  are  seen.  There  are  also 
occasional  lenses  consisting  of  a  central  mass,  probably  rhodochrosite 
(MnC02),  with  secondary  muscovite  at  both  ends,  measuring  in  all  0.14 
millimeter  in  length.  There  are  not  a  few  prisms  of  apatite  0.025 
millimeter  long.  No  other  carbonates  are  present,  nor  is  there  any 
effervescence  with  cold  dilute  hydrochloric  acid. 
This  is  a  very  superior  quality  of  slate.  Its  ready  fissility,  attract- 
ive color,  and  the  absence  of  calcium  and  magnesium  carbonate  all 
commend  it. 
Conclusion. — While  all  but  one  of  these  slates  have  been  denomi- 
nated phyllites  on  account  of  sericitization  of  the  matrix,  they  all 
show  clastic  quartz.  The  remarkably  fine  cleavage  and  the  absence  of 
calcium  and  magnesium  carbonate  in  the  black  (1)  and  the  green  (8) 
render  them  exceptionally  good.  The  reddish  slate  (1)  is  good  and 
(3)  may  prove  equally  so.  If  (1)  and  (6)  occurred  in  a  populous  region 
they  would  doubtless  be  in  great  demand.  For  commercial  purposes 
the  microscopic  examinations  of  (1)  and  (6)  ought  to  be  supplemented 
by  partial  chemical  analyses  to  show  whether  they  are  entirely  free 
from  carbonate,  as  the  microscope  indicates. 
