LAKE  SUPERIOR  REGION.  109 
erly  sides  are  invariably  tilted  to  a  high  angle  near  the  range  of  hills. 
The  sedimentary  rocks  on  the  north  are  traversed  by  numerous  dikes, 
varying  in  thickness  from  50  to  400  or  500  feet.  The  trap  rocks  on 
this  northwestern  escarpment  were  in  an  intense  state  of  ignition 
while  in  contact  with  the  sedimentary  rocks,  as  shown  by  the  great 
changes  which  the  sedimentary  rocks  have  undergone.  The  author 
is  disposed  to  regard  the  amygdaloid  as  due  to  the  fusion  of  the  lower 
portions  of  the  sedimentary  rocks. 
At  Presque  Isle  is  a  little  isolated  knob  of  trap  which  has  been  up- 
lifted, as  is  shown  by  the  way  in  which  the  stratification  of  the  adja- 
cent sedimentary  rocks  has  been  disturbed.  They  invariably  dip  at  a 
high  angle  in  all  directions  from  the  trap.  At  the  immediate  line  of 
junction  the  character  of  both  rocks  is  lost,  and  the  sedimentary  rocks 
for  a  distance  of  several  hundred  feet  have  been  shattered  while  re- 
taining their  original  position,  and  were  again  cemented  by  an  injec- 
tion of  calcareous  matter. 
The  area  of  country  occupied  by  the  metamorphic  group  is  less  than 
by  the  Primary  or  trap.  The  group  is  made  up  of  an  alternating 
series  of  talcose  and  mica  slates,  graduating  into  clay  slates,  with 
quartz  and  serpentine  rocks,  the  quartz  rocks  being  by  far  the  most 
abundant.  The  metamorphic  rocks  are  occasionally  traversed  by  trap 
dikes. 
The  conglomerate  rock,  the  lowest  of  the  sedimentary  rocks,  is  inva- 
riably connected  with  or  rests  upon  the  trap  rock.  It  is  very  variable 
in  thickness  and  is  without  doubt  a  trap  tuff  which  has  accumulated  or 
been  deposited  around  the  conical  knobs  of  traps  during  their  gradual 
elevation.  The  pebbles  of  the  rock  consist  of  rounded  masses  of 
greenstone  and  amygdaloid.  They  are  usually  firmly  cemented  by 
calcareous  and  argillaceous  material.  Resting  conformably  upon  the 
conglomerate  is  a  mixed  conglomerate  and  sand  rock.  This  mixed 
rock  occurs  upon  Isle  Royal  and  was  seen  to  be  very  widespread  on 
the  south  shore.  The  conglomeratic  part  of  the  mixed  rock  has  the 
same  character  as  the  conglomerate  rock.  Dikes  of  greenstone  are 
found  in  this  mixed  rock,  but  less  frequently  than  in  the  rock  below. 
The  red  sandstone  is  the  chief  rock  that  appears  upon  the  immediate 
coast  of  the  south  shore  of  Lake  Superior.  The  Primary,  metamor- 
phic, and  trap  rocks  are  almost  invariably  surrounded  or  flanked  at 
their  bases  by  this  sand  rock.  The  material  of  this  sand  rock  differs 
widely  from  the  conglomerate  rocks,  for  these  are  made  up  of  ma- 
terials clearly  of  trappean  origin  and  very  rarely  of  quartz,  while  the 
red  sandstone  is  composed  of  materials  derived  from  the  granitic  and 
metamorphic  rocks,  in  which  quartz  occurs  abundantly.  The  red 
sand  rock  is  less  frequently  traversed  by  dikes  than  the  rocks  before 
described,  although  they  are  sometimes  noticed  traversing  the  whole 
of  the  several  rock  formations,  including  the  red  sandstone,     The 
