116  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  200. 
impervious  and  decomposed,  and  the  deposition  localized  along  the 
dike.  In  this  class  of  veins,  also,  the  ore  shoots  in  detail  are  often 
due  to  the  juncture  of  branches  with  the  main  vein. 
Influence  of  the  wall  rocks.— The  nature  of  the  country  rock  along 
the  line  of  fracture  of  a  vein  influences  greatly  the  character  of  the 
fracture  formed,  and  consequently  the  nature  of  the  ore  depositee 
in  the  vein.     Brittleness  seems  to  be  the  especial  quality  which  enable* 
rocks  to  form  strong  fractures.     Of  all  the  rocks  of  the  district  the 
granites  and  the  hard  gneisses  as  a  rule  answer  this  qualification  best 
On  the  basis  of  this  theory  we  might  expect  to  find  the  broadest  an< 
strongest  veins   (not  necessarily  with  the  highest  grade  of  ore)   in 
these  rocks.     The  facts  tend  to  uphold  the  theory  in  the  case  of  thu 
strong  fissure  veins  of  the  Mendota-Frostberg  system  and  also  i 
otheAnstances.     Highly  porphyritic  granite  seems  especially  like! 
to  occur  as  a  wall  rock  of  veins.     The  altered  form  of  this  granitii 
occurring  in  the  vicinity  of  veins  has  white  altered  phenocrysts.    II 
this  altered   form   the  rock  is  locally   termed   A  corn   rock,"   and     j 
regarded  by  the  miners  as  a  u  good  rock  for  ore."     This  porphyria 
phase,  which  may  represent  more  rapid  cooling  than  in  the  ordinaij 
granite,  occurs  near  the  contacts  of  the  more  granular  masses  and  alii 
rn  numerous  dikes.     It  is  thus  frequently  in  contact  with  gneiss  < 
other  rocks  of  less  or  at  least  different  resistance  to  strain.     Then;, 
fore,  as  in  the  case  of  porphyry  dikes,  fault  movements  have  take!;, 
place  by  preference  along  these  contacts.    This  consideration,  togethi 
with  that  of  the  natural  rigidity  of  the  porphyritic  granite,  explai 
its  frequency  as  a  wall  rock  of  strong  veins. 
A  fissile  gneiss  usually  gives  rise  to  a  much  weaker  vein,  whil 
soft  micaceous  gneiss  or  schist  is  exceptionally  poor  for  vein  form 
tion.  Many  cases  were  noted  where  strong  veins  in  granite  up 
entering  soft  micaceous  gneiss  pinched  to  a  faint  slip  or  else  scatter 
and  disappeared  entirely,  by  sending  ramifying  branches  betwe 
the  laminae  of  the  gneiss. 
As  far  as  observations  could  detect,  the  porphyry  of  the  distr 
did  not  seem  to  have  any  influence  upon  the  formation  of  the  < 
deposits  other  than  that  exerted  by  the  other  rocks  of  the  regi 
The  relations  between  the  veins  and  the  porphyry  dikes  all  tend! 
show  that  the  porphyry  dikes  were  not  only  well  consolidated  lc 
before  the  veins  were  formed,  but  that  in  many  cases  they  had  und 
gone  considerable  alteration  prior  to  the  fracturing.  As  a  gene 
rule  the  veins  of  the  district  do  not  "  make  "  much,  ore  when  porph; 
forms  both  walls  of  the  fracture,  In  many  cases  a  strong  lead 
granite  or  hard  gneiss  dwindles  in  size  while  passing  through  j 
phyry,  which  is  usually  somewhat  soft  from  alteration.  This  u 
cates  that  the  ores  were  deposited  after  the  softening  of  the  porph 
X 
