480  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
South  of  the  road  between  Madoc  and  Bridgewater  there  is  evidence 
of  a  considerable  break  between  the  conglomeratic  rocks  and  the  lime- 
stones and  associated  hornblende  schists  and  felsites.  Southward  the 
conglomerate  gradually  becomes  coarser  and  near  its  base  contains 
very  numerous  fragments  of  hornblende  schist,  felsite,  and  limestone 
which  unquestionably  have  been  derived  from  these  formations  just 
to  the  south.  It  is  thus  clear  that  there  is  in  this  area  a  considerable 
break  between  the  two  parts  of  the  sedimentary-  series.  Miller  and 
Knight  hold  that  both  parts  carry  limestone,  making  discrimination 
on  a  lithologic  basis  difficult.  The  upper  of  the  two  series  they  call 
Hastings  and  correlate  with  the  Huronian,  and  the  lower  with 
the  Keewatin.  Adams  and  Barlow  do  not  discriminate  the  two 
series,  but  regard  the  Hastings  as  a  less  metamorphosed  part  of  the 
Grenville,  as  it  is  typically  exposed  in  the  original  Laurentian  dis- 
trict. How  general  the  break  is  can  be  determined  only  by  further 
study. 
As  to  the  relations  which  obtain  between  the  granite  gneiss  series 
and  the  sedimentary  series  two  questions  need  consideration — first, 
its  relations  to  the  upper  sedimentary  series,  and,  second,  its  relations 
to  the  lower  series.  Adams  and  Barlow  believed  that  the  granite 
gneiss  series  has  batholithic  relations  with  the  limestone  series,  and 
as  they  did  not  discriminate  the  two  series,  they  supposed  this  relation 
to  obtain  for  both.  However,  the  presence  of  abundant  granite  and 
gneiss  pebbles  of  various  kinds  in  the  conglomerate  of  the  upper 
series  leads  to  the  conclusion  that  the  granite  gneiss  series  was  present 
before  the  deposition  of  the  conglomerate,  and  if  this  be  so  it  empha- 
sizes the  importance  of  the  break  in  the  sedimentary  formations  noted 
near  Madoc  and  Bridgewater. 
At  one  place  in  the  district  the  international  committee  of  geolo- 
gists found  an  area  of  calcitic  and  chloritic  ellipsoidal-weathering 
schists  which  resemble  the  Keewatin  of  Lake  Superior.  Miller  and 
Knight  later  found  the  "  Grenville  "  limestone  to  rest  upon  basic 
igneous  flows. 
Felsite,  felsite  breccia,  and  other  volcanics  are  associated  with  the 
sedimentary  rocks.  These  are  extensive  in  area  and  are  an  important 
feature  of  the  district.  Also  associated  with  the  Hastings  series  are 
abundant  augitic  and  hornblendic  rocks,  many  of  which  are  probably 
eruptive. 
OUTLYING  DISTRICTS. 
The  outlying  districts  have  not  been  studied  to  the  same  extent  as 
the  original  Laurentian  and  Hastings  districts.  However,  it  is 
known  for  the  outlying  areas  as  wholes  that  the  predominating  rocks 
are  granites  and  gneisses  similar  to  the  Ottawa  gneiss  of  the  original 
Laurentian  district  and  are  to  be  classed  as  Laurentian  under  the 
