304  PRE-CAMBRIAN   GEOLOGY   OF   NORTH   AMERICA. 
the  same  significance  as  in  sedimentary  series,  they  are  placed  as  the 
earliest  of  the  Keweenawan  flows.  The  thickness  of  the  lower  divi- 
sion is  placed  in  round  numbers  at  25,000  to  30,000  feet,  while  at 
Montreal  River  its  apparent  thickness  is  33,000  to  35,000  feet,  but  a 
part  of  this  may  be  due  to  the  westward  continuation  of  the  Kewee- 
naw fault. 
Detailed  descriptions  are  given  of  the  rocks  of  Keweenaw  Point,  of 
the  region  between  Portage  Lake  and  Ontonagon  River,  of  the  South 
range,  of  the  region  between  Ontonagon  River  and  Numakagon  Lake, 
including  the  Porcupine  Mountains,  of  northwestern  Wisconsin  and 
the  adjoining  part  of  Minnesota,  of  the  Minnesota  coast,  of  Isle  Royal 
and  Nipigon  Bay,  and  of  Michipicoten  Island  and  the  east  coast  of 
Lake  Superior.  Silver  Mountain,  belonging  to  the  South  range,  is 
composed  of  diabase,  dipping  at  an  angle  of  30°,  and  appears  to  be 
surrounded  by  horizontal  sandstone.  On  the  west  branch  of  the  On- 
tonagon are  found  cliffs  of  horizontal  sandstone  almost  in  proximity 
with  ferruginous  slate  supposed  to  belong  to  the  Huronian,  and  but  a 
short  distance  from  diabases  regarded  as  Keweenawan.  The  isolated 
position  of  the  South  range  is  regarded  as  due  to  a  fault,  as  there  is 
no  evidence  whatever  of  a  fold,  and  to  regard  this  part  of  the  series 
as  a  continuous  conformable  succession  with  the  Keweenawan  rocks 
to  the  north  would  give  the  series  an  incredible  thickness.  The  Por- 
cupine Mountains  are  found  to  be  due  to  a  subordinate  fold  in  the 
series,  the  core  being  a  quartz  porphyry. 
All  the  known  facts  with  reference  to  the  relations  of  the  horizontal 
sandstone  to  the  Copper-bearing  rocks  of  northwestern  Wisconsin  and 
the  adjoining  parts  of  Minnesota  are  recapitulated.  The  unconform- 
ity between  the  fossiliferous  Cambrian  of  the  St.  Croix  Valley  and 
the  bedded  melaphyre  and  amygdaloids  described  by  Sweet,  Strong, 
and  Chamberlin  is  indisputable,  and  the  latter  rocks  are  identical  in 
nature  and  in  structure  with  the  similar  rocks  of  Keweenaw  Point  and 
have  been  shown  to  be  in  actual  continuity  with  them.  At  Snake  and 
Kettle  rivers  the  diabase  and  diabase  amygdaloids  with  interbedded 
porphyry  conglomerates  are  in  all  respects  like  those  of  Keweenaw 
Point,  and  here,  as  shown  by  Chamberlin  and  McKinlay,  the  horizon- 
tal Cambrian  sandstone  overlies  these  beds  unconformably.  Sweet's 
examination  of  Kettle  and  St.  Croix  rivers  shows  that  here  are  cu- 
priferous rocks  which  are  identical  with  those  of  Keweenaw  Point, 
upon  which  the  red  sandstone  of  Lake  Superior  west  of  the  Montreal 
reposes  unconformably,  at  Black  River,  Copper  Creek,  Aminicon 
River,  and  Middle  River.  The  disturbances  of  the  overlying  sand- 
stone described  by  Sweet  are  due  in  part  to  the  irregularities  of  an 
unconformable  contact  and  to  the  pressure  of  the  deep-seated  Kewee- 
naw rocks  against  the  more  shallow  sandstone,  but  also  in  large  meas- 
ure to  a  faulting  that  has  taken  place  along  the  contact  line.     The 
