134  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1903.  [bull. 225. 
tricts  in  their  general  direction,  but  exhibit  a  very  uniform  trend 
within  the  limits  of  any  single  productive  area.  These  channel-like 
ore  bodies  are  known  ay  shoots,  and  have  a  width  of  from  a  few  inches 
to,  in  rare  instances,  300  feet.  The  average  width  is  perhaps  about 
30  feet,  although  all  widths  between  5  and  100  feet  are  of  frequent 
occurrence.  The  length  is  in  all  cases  many  times  in  excess  of  the 
breadth,  and  in  the  case  of  the  Tornado-Mogul  shoot  is  about  three- 
fourths  of  a  mile.  The  vertical  dimensions  vary  from  a  few  inches 
to  a  maximum  of  18  feet.  The  average  thickness  is  about  (3  feet. 
The  shoots  generally  follow  either  single  fractures  which  are  parallel 
to  their  longer  diameter  or  broad  areas  of  parallel  or  intersecting 
fractures.  The  beds  of  rock  that  lie  above  the  ore  are  generally  shale 
of  a  more  or  less  impervious  character,  but  sills  of  eruptive  rock  not 
inf  requentty  play  the  same  role.  On  the  lower  contact  the  floor  is 
sometimes  of  basal  Cambrian  quartzite,  but  in  many  cases  varying 
thicknesses  of  dolomite  intervene  between  the  quartzite  and  the  ore.  In 
such  cases  the  widest  portion  of  the  shoot  is  directly  beneath  the  imper- 
vious rock  of  the  roof,  for  the  solutions  have  spread  out  and  replaced 
dolomite  for  a  great  distance  along  the  under  surface  of  the  impervious 
rock.  The  shoots  have  thus  a  wedge-shaped  form  in  many  cases,  the 
broadest  portion  of  the  wedge  being  at  the  top. 
The  fractures. — When  the  ore  that  forms  the  bod}^  of  a  shoot  has 
been  removed  the  fractures   b}^  which  the  mineralizers  have  gained 
access  to  the  rock  replaced  may  be  traced  in  the  overlying,  and  where 
they  are  uncovered,  in  the  underlying  beds.     These  fractures  have 
been  rendered  prominent  by  a  slight  siliciiication  of  the  adjoining  rock, 
which  has  often  caused  them  to  project  from  the  softer  shaly  material; 
they  are  frequently  iron  stained.     These  silicified,  iron-stained  frac- 
tures  are  commonly  known  as  ''verticals."     They  may  be  observed  in 
greater  or  less  number  in  all  the  shoots  of  the  refractory  siliceous  ore.  j 
The  fractures  are   generally  slightly   warped  surfaces   along   which 
a  little  movement  has  occurred,  or  they  may  be  composite  zones  of  I 
fracture  caused  by  the  intersection  of  many  small,  irregular  fissures. 
The  displacement  along  such  planes  of  movement  is  generally  very 
small — not  more  than  2  or  3  inches — but  it  sometimes  reaches  6  or  7  J 
feet.     They  are  usually  without  appreciable  open  space,  for  the  walls 
have  not  generally  been  removed   from  one   another   for   distances! 
greater  than  one  sixty-fourth  of  an  inch.     Some  notable  exceptions] 
occur.     The  verticals  frequently  extend  into  the  beds  that  form  the' 
roof  of  the  ore  bodies,  but  sometimes  terminate  in  the  ore-bearing 
beds   themselves.      They  have  also  been  traced  through  the  lower 
quartzite  into  the  Algonkian  below,  but  on  entering  that  formation 
their  traces  are  lost  in  the  vertical  lamina;  of  the  schists. 
Productive  areas. — The  productive  areas  of  refractory  siliceous  ore! 
