GENERAL  ACCOUNT    OF    THE    PRE-CAMBEIAN    ROCKS.  39 
amounts  of  sediments  is  striking.  Anyone  who  has  seen  the  evenly 
bedded  Algonkian  sediments  resting  upon  the  erosion  edges  of  the 
gnarled  and  contorted  complex  of  Archean  schists,  gneisses,  and  other 
rocks  would  not  question  the  conclusion  that  this  unconformity  is  a 
great  one. 
Similar  statements  may  be  made  for  the  Lake  Huron  region  and 
for  the  great  northern  interior  of  Canada,  although  here  detailed 
mapping  has  covered  but  limited  areas. 
Locally  in  both  of  the  regions  deformation  has  nearly  or  quite  ob- 
literated the  unconformity.  In  places  also  the  basal  member  of  the 
Algonkian  is  a  little-assorted  conglomerate  made  up  of  fragments 
like  the  immediately  underlying  rock.  Some  of  the  greenstone  con- 
glomerates of  this  horizon,  composed  of  fragments  of  the  underlying 
greenstone,  are  distinguished  with  great  difficulty  from  the  tuffaceous 
phases  of  the  underlying  greenstone. 
For  New  Brunswick  and  Cape  Breton  the  unconformity  between 
the  Algonkian  Grenville  and  the  Archean  has  not  been  determined, 
though  it  doubtless  exists. 
In  Newfoundland  there  is  a  strong  unconformity  betAveen  the 
Algonkian  Avalon  and  the  Laurentian  gneisses. 
The  only  district  in  eastern  United  States  where  there  has  been  an 
attempt  to  separate  Algonkian  and  Archean  rocks  by  an  uncon- 
formity is  in  the  southern  Appalachians,  where  certain  effusive  rocks 
are  classed  as  Algonkian,  while  the  much  deformed  and  metamor- 
phosed plutonic  rocks  upon  which  they  rest  are  called  Archean. 
This  is  an  unconformity  between  eruptive  rocks,  which  may  be  of 
an  order  different  from  that  of  the  unconformities  above  discussed. 
In  the  Grand  Canyon  of  the  Colorado  the  unconformity  is  striking. 
Here  evenly  bedded  Algonkian  sediments  may  be  seen  resting  upon 
a  nearly  even  erosion  surface  truncating  gneisses  and  schists  of  the 
greatest  lithological  variety  and  intricacy  of  structure. 
In  the  Front  Range  of  Colorado  and  in  the  Medicine  Bow  Moun- 
tains of  Wyoming,  and  elsewhere  in  these  States,  an  unconformity 
between  the  Algonkian  and  the  Archean  is  known,  but  it  is  not  con- 
spicuous. The  deformation  which  the  rocks  have  undergone  has  de- 
veloped a  parallel  schistosity  in  the  Algonkian  and  Archean  rocks 
which  has  largely  obscured  the  true  relations. 
The  quartzite  of  the  Wasatch  Mountains  of  Utah,  provisionally 
assigned  to  the  Algonkian,  rests  upon  the  Archean  with  tremendous 
unconformity.  The  Belt  series  of  Montana  also  is  unconformable 
upon  the  Archean  with  marked  discordance. 
In  various  other  parts  of  the  West  unconformities  between  the 
Algonkian  and  the  Archean  have  been  inferred,  but  not  actually 
observed. 
