92  .  -     PEE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA..  [bull. 86. 
often  conglomeratic j  XY  is  argillite  or  clay  slate;  XVI  is  uncertain, 
it  contains  some  soft  hematite;  XVII  is  anthophyllitic  schist,  contain- 
ing iron  and  manganese;  XVIII  is  doubtful;  XIX  is  mica-schist,  con- 
taining staurolite,  andalusite,  and  garnets.  The  total  thickness  of  the 
whole  Marquette  series  maj^have  an  aggregate  of  5,000  feet. 
The  beds  appear  to  be  metamorphosed  sedimentary  strata,  having 
many  folds  or  corrugations,  thereby  forming  in  the  Marquette  region 
an  irregular  trough  or  basin,  which,  commencing  on  the  shore  of  lake 
Superior,  extends  west  more  than  40  miles.  The  upturned  edges  of 
these  rocks  are  quite  irregular  in  their  trend  and  present  numerous  out- 
crops. While  some  of  the  beds  present  lithological  characters  so  con- 
stant that  they  can  be  identified  wherever  seen,  others  undergo  great 
changes.  Marble  passes  into  quartzite,  which  in  turn  graduates  into 
novaculite. 
Xear  the  junction  of  the  Huronian  and  Laurentian  systems,  in  the 
Marquette  region,  are  several  varieties  of  gneissic  rocks,  composed  in 
the  main  of  crystalline  feldspar,  with  glassy  quartz  and  much  chlorite. 
Intersecting  these  are  beds  of  hornblendic  schist,  argillite,  and  some- 
times chloritic  schist.  These  rocks  are  entirely  beneath  all  of  the  iron 
beds,  seem  to  contain  no  useful  mineral  or  ores,  and  are  of  uncertain 
age.    Xo  attempt  is  here  made  to  describe  or  classify  them. 
The  diorites,  dioritic  schists'and  related  rocks  range  in  structure  from 
very  fine  grained  or  compact  (almost  aphanite)  to  coarsely  granular 
and  crystalline,  being  sometimes  porphyritic  in  character.  The  rock 
passes  on  the  one  hand  into  a  hornblende  rock,  and  on  the  other  into  a 
rock  resembling  a  diorite.  It  is  eminently  schistose  in  character,  split- 
ting easily,  and  appearing  more  like  chloritic  schist  than  any  other 
rock.  At  several  points  dioritic  schists,  semi-amygdaloidal  in  char- 
acter, were  observed,  and  in  one  instance  the  rock  had  a  strong  resem- 
blance to  a  conglomerate.  The  bedding  of  the  rocks  is  generally  obscure 
and  sometimes  entirely  wanting.  It  is  only  by  a  full  study  of  the  rock 
in  mass  and  its  relations  to  the  adjacent  beds  that  one  becomes  con- 
vinced, whatever  its  origin,  that  it  presents  in  mass  precisely  the  same 
phenomenon  as  regards  stratification  as  do  the  accompanying  schists 
and  quartzites.  Chloritic  magnesian  schists  are  associated  with  the 
pure  and  mixed  ores.  Oftentimes  these  magnesian  schists  several  feet 
in  width  cut  across  the  stratification  and  are  called  slate  dikes.  It  is  dif- 
ficult or  impossible  to  draw^  the  line  between  these  magnesian  schists 
and  the  dioritic  schists.  It  is  suggested  that  on  the  New  England-Sag- 
inaw range  and  at  the  lake  Superior  mine,  tepid  alkaline  waters  have 
penetrated  the  formation  and  have  dissolved  out  the  greater  portion  of 
the  siliceous  matter,  leaving  the  iron  oxide  in  a  hydrated  earthy  condition, 
At  the  S.  C.  Smith  mine  and  along  Plumbago  brook  is  found  carbon- 
aceous matter.  These  carbonaceous  shales  burn  white  before  the  blow- 
pipe and  mark  paper  like  a  piece  of  charcoal. 
Above  the  Cascade  ore  is  a  bed  of  coarse  conglomerate.     The  upper 
