136  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
fragments  of  the  porphyry  conglomerate,  with  which  rock  the  con- 
tact here  occurs,  are  found  to  dip  at  a  considerable  angle  away  from 
the  eruptive  rock  and  to  rest  directly  upon  it.  At  the  St.  Louis  ra- 
vine the  sandstone  is  found,  upon  approaching  the  Keweenaw  series, 
to  become  rapidly  tilted  upward,  and  before  reaching  the  Kewee- 
nawan  rocks  to  become  vertical,  while  the  interstratified  eruptives  and 
detrital  rocks  of  the  Keweenaw  series  dip  away  from  the  sandstone. 
At  the  Douglass  Houghton  ravine  the  horizontal  sandstone  is  found, 
upon  approaching  the  Keweenaw  series,  to  become  bent  into  a  series 
of  folds  and  to  dip  downward  under  the  traps  and  porphyries,  which 
dip  at  a  steeper  angle  in  the  same  direction.  Along  the  contact  the 
trap  is  shattered.  At  the  Torch  Lake  quarry  the  sandstone  is  found 
to  be  in  a  horizontal  position,  there  being  found  no  evidence  what- 
ever that  this  structure  is  jointing  or  that  the  real  dip  has  an  inclina- 
tion, as  described  by  Wadsworth.  The  crystal  outlined  grains  of 
sand  here  contained  are  found  to  be  produced  by  secondary  growth 
rather  than  crystals  derived  from  quartz  porphyry.  At  the  Hun- 
garian ravine  the  relations  are  much  the  same  as  those  at  the  Doug- 
lass Houghton  ravine,  except  that  the  Keweenawan  diabase  is  inter- 
stratified with  conglomerate  instead  of  quartz  porphyry.  Along  the 
contact  the  sandstone  is  broken. 
In  getting  at  an  explanation  of  the  facts  there  must  be  taken  into 
account  the  bedded  nature  of  the  Keweenaw  series;  the  uniformity 
and  steadiness  of  dip ;  the  enormous  thickness  of  the  Keweenaw  series ; 
the  general  horizontality  of  the  Eastern  sandstone;  the  quartzose 
character  of  the  Potsdam  sands  in  distinction  to  the  silicate  nature  of 
the  Keweenawan  sands;  the  mutual  relations  and  distribution  of  the 
two  series ;  the  relations  to  topography ;  the  relations  of  the  two  series 
to  drainage;  the  comparative  straightness,  but  with  gentle  undula- 
tions, of  the  junction  line  throughout  its  course  of  nearly  100  miles; 
the  coincidence  of  the  line  of  escarpment  with  the  line  of  junction  of 
the  two  series;  the  disturbance  along  the  line  of  contact;  the  special 
character  of  the  distortions;  the  character  of  the  junction;  the  junc- 
tion debris ;  the  irregular  and  broken  contact  f acies  of  the  two  series ; 
the  fact  that  the  contact  occurs  between  the  Eastern  sandstone  and 
different  members  of  the  Xveweenaw  series;  the  discordance  of  strike; 
the  derivation  of  the  pebbles  of  the  Eastern  sandstone  from  the 
Keweenaw  series;  the  distribution  of  the  pebbles,  those  of  the  Kewee- 
naw series  being  found  only  near  the  immediate  junction;  the  imper- 
fect assortment  of  the  pebbles  and  matrix  near  the  junction ;  the  angu- 
larity of  the  pebbles  at  this  place;  the  absence  of  large  fallen  masses 
of  trap  in  the  Eastern  sandstone ;  and  the  proximity  and  relations  of 
the  Trenton  limestone,  resting  as  it  does  upon  the  Eastern  sandstone 
within  a  short  distance.  These  specifications  are  taken  to  point  with 
distinctness  to  the  conclusion  that  the  Keweenaw  series  is  much  older 
