48  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
tinctly  regarded  as  an  exception  to  the  ordinary  stratigraphieal  laws 
and  separated  both  in  descriptions  and  mapping.  This  is  the  more 
creditable  because  for  many  years  afterwards  similar  rocks  were  sup- 
posed by  many  other  writers  to  be  parts  of  the  stratified  successions  in 
a  completely  metamorphosed  condition  had  caused  them  to  become 
crystalline.  Finally  it  was  recognized  that  this  Huronian  series  rests 
unconformably  upon  an  older  gneissic  and  granitic  crystalline  series, 
which  has  yielded  abundant  fragments  to  the  overlying  rocks. 
To  the  person  who  hypothecates  that  all  pre-Oambrian  rocks  are 
wholly  crystalline  and  that  all  truly  detrital  rocks  are  Cambrian  or 
post-Cambrian,  these  conclusions  prove  only  that  inferior  to  the  Pots- 
dam sandstone,  and  separated  from  it  by  a  great  unconformity,  is  a 
series  of  rocks  of  great  thickness,  having  a  number  of  persistent  mem- 
bers of  varying  lithological  character,  which  are  lower  Cambrian.  But 
this  position  does  not  lessen  the  value  of  the  work  done;  for  it  would 
still  be  true  that  the  Huronian  series  was  the  first  so  low  in  the  geolog- 
ical column  in  which  the  above  facts  were  shown. 
The  only  rocks  in  the  Original  Huronian  area  which  Logan  correlated 
with  those  of  the  Original  Laurentian  area  are  the  unconformably  under- 
lying granitic  and  gneissic  series.  These  were  called  lower  Laurentian, 
the  idea  being  evidently  to  correlate  them  with  the  lower  non calcareous 
division  of  the  Original  Laurentian.  This  correlation  was  plainly  made 
on  the  ground  of  lithological  likeness.  That  the  Huronian  is  more 
recent  than  the  upper  Laurentian  was  stated  only  as  a  belief.  This 
belief  appears  to  have  been  based  upon  the  "nonmetamorphie"  char- 
acter of  the  Huronian  as  compared  with  the  upper  Laurentian.  It  is 
also  possible  that  the  fact  that  there  is  a  structural  break  on  the  north 
shore  of  lake  Huron,  between  the  Huronian  and  the  gneissic  series, 
whereas  no  such  break  was  found  between  the  upper  and  lower  divis- 
ions of  the  Original  Laurentian,  had  an  influence  in  leading  to  this 
conclusion. 
NOTES. 
The  following  are  the  titles  of  papers  and  works  cited  in  the  forego- 
ing chapter,  reference  marks  to  which  appear  in  the  text: 
1  On  the  Geology  of  the  Ottawa  and  some  of  its  Tributaries,,  W.  E.  Logan.  Rept.  of 
Prog.  Geol.  Survey  of  Canada  for  1845-46,  pp.  40-51. 
2  On  the  Geology  of  the  Counties  of  Beauharnois  and  the  Lake  of  Two  Mountains, 
W.  E.  Logan.     Rept.  of  Prog.  Geol.  Survey  of  Canada  for  1851-52. 
3  On  the  Geology  of  the  Region  between  the  Ottawa,  the  St.  Lawrence,  and  the 
Rideau,  Alexander  Murray.     Ibid.,  pp.  59-65. 
4  W.  E.  Logan :  Rept.  of  Prog.  Geol.  Survey  of  Canada  for  1852-'53,  pp.  8,  74. 
5  On  the  Geology  of  the  Region  between  Kingston  and  Lake  Sirncoe,  Alexander 
Murray.     Ibid.,  pp.  75-133. 
6  On  the  Laurentian  rocks  of  Grenville,  Chatham,  St.  Jerome,  etc.,  and  the  Eco- 
nomic Materials  found  in  them,  W.  E.  Logan.  Rept.  of  Prog.  Geol.  Survey  of  Canada 
for  1853-54-55-56,  pp.  5-57.     Accompanied  by  a  sketch  map. 
7  On  the  Topographical  and  Geological  Features  of  the  Region  between  the  Ot- 
tawa river  and  Georgian  Bay,  as  well  as  North  of  Lake  Huron,  Alexander  Murray. 
Ibid.,  pp.  59-190.    Accompanied  by  two  maps. 
