?6  PRE-CAMRKIAX    ROCKS    OF    NORTH    AMERICA.  [bull. 88. 
sedimentary  strata  have  undergone  great  change  by  the  action  of  the 
trap  rocks.  Along  the  line  of  the  junction  a  chemical  combination  of 
The  materials  of  the  sandstones  and  trap  rocks  took  place,  forming  the 
vesicular  trap  called  amygdaloid,  while  there  has  further  resulted  from 
this  action  a  breccia  ted  or  trap  turf,  consisting  of  broken  pieces  of 
amygdaloid  and  sandstone.  At  other  times  the  sandstone  is  indurated 
into  a  flinty  red  rock  resembling  jasper.  At  the  Copper  fails  mine  is 
a  ease  of  what  appears  to  be  an  Orthocera  in  the  breccia  of  amygdaloids 
and  altered  limestones  May  it  not  have  been  torn  from  a  subjacent 
bed  of  Silurian  limestone  by  the  agency  of  the  intruded  trap  rocks  1 
At  the  coast  off  lac  la  Belle  the  sandstone  in  contact  with  trap  has  a 
dip  to  the  south  of  30°  while  at  point  Isabelle  the  sandstone  cliffs  are 
nearly  horizontal. 
Locke"  rinds  near  L'Anse  that  the  trappean  rocks  contain  frag- 
ments ^>i  slate  distributed  through  it  and  converted  into  a  hornstone 
when  in  small  pieces,  like  the  eruptive  granite  of  Pigwacket  mountain. 
Xew  Hampshire.  At  point  No.  2.  west  of  Presque  isle,  is  a  junction  of 
3  I'idsrone  and  syenite.  The  mass  of  syenite  intersected  by  dikes 
of  trap  is  under  the  sandstone  and  seems  to  have  but  slightly  affected 
it. 
Foster  and  Whepkey"  accompany  their  synopsis  of  their  explora- 
tion- by  g        _    al  maps  of  the  region  between  Portage  lake  and  the 
Montreal  ri\  ei.  Keweenaw  point,  isle  Royale.  and  the  region  bet 
Keweenaw  bay  and  Chocolate  river. 
Whitney -T  finds  the  rocks  in  the  district  between  Portage  lake  and 
the  Ontonagon  river  t<:>  comprise  the  following:  First,  the  red  sand- 
stone of  lake  Superior,  the  age  of  which  can  not  be  determined,  as  it  is 
destitute  of  fossils.  It  lies  directly  upon  the  granitic  rocks.  Second,  a 
bed  of  loss  liferous  lime-t<me  of  the  Lower  Silurian  system,  which 
cs  in  an  isolated  position  and  has  but  a  limited  extent,  and  the 
relations  of  which  to  the  sandstone  have  not  been  determined  with  cer- 
tainty. Third,  the  trappean  rocks.  Fourth,  granitic  and  syenitic 
rocks,  with  hornbleude  and  greenstone.  The  farther  the  red  sand- 
stone is  removed  from  the  trappean  rocks,  so  much  nearer  do  its  strata 
approach  to  the  horizontal  and  also  become  lighter  color  and  more 
fragile.  The  conglomerate  of  Keweenaw  point  occurs  mixed  and  inter- 
calated in  such  a  manner  with  the  sandstone  as  to  leave  no  doubt  of 
their  common  origin  and  identity  of  age.  In  general  the  beds  of  eon- 
glomerate  increase  in  frequency  in  nearing  the  trap.  The  sand-tone 
does  not  repose  directly  on  the  trap,  bnt  almost  invariably  a  bed  of 
se  pebble  rock  is  found  interposed  between.  A  trap  breccia  found 
at  Cushmaifs  -e^ms  to  be  a  product  of  the  interfusion  of  trap  and 
sandstone.  Compact  quartz  rock  or  jasper  occurs  abundantly  in  moun- 
tain masses  in  the  Porcupine  mountains. 
Whitney  describes  a  deposit  of  limestone  which  rises  to  a  height  of 
about  300  feet  above  the  general  level  of  the  country  near  L'Anse. 
