SUMMARY   OF    GENERAL   LITERATURE.  103 
ceptable  to  the  Canadian  geologists,  who  do  not  regard  the  term  as 
necessary,  as  well  as  to  the  American  geologists,  who  regard  the  Kee- 
watin  as  Archean,  unconformably  below  the  Algonkian. 
Daly,42  in  1907,  concludes  that  the  lime  salts  of  the  pre-Cambrian 
ocean,  inherited  from  Azoic  time,  were  precipitated  as  calcium  car- 
bonate comparatively  soon  after  the  introduction  of  animal  life  into 
the  sea. 
During  most  of  Eozoic  time — that  is,  pre-Cambrian  time  in  which 
animal  life  existed — the  ocean  was  practically  limeless;  calcareous 
secretions  by  animals  were  impossible. 
Tests  and  skeletons  of  pure  chitin  were  possible  in  Eozoic  time, 
but  were  not  abundantly  preserved  until  some  carbonate  or  phos- 
phate of  lime  was  built  into  those  structures.  The  calcareo-chitinous 
tests  of  Cambrian  and  Ordovician  trilobites  and  shells  of  brachiopods 
represent  a  transition  stage  between  the  Eozoic  eon  of  dominantly 
soft-bodied  animals  and  the  post -Cambrian  eon  of  dominantly  lime- 
secreting  animals.  The  notable  fossilization  of  brachiopods,  trilo- 
bites, mollusks,  etc.,  was  impossible  until  near  the  beginning  of  Cam- 
brian time.  Indeed,  the  conditions  for  truly  abundant  fossilization 
of  calcareous  forms  were  not  established  until  after  the  Cambrian 
period.  The  striking  variety  or  entire  lack  of  organic  remains  in 
the  thick  Cambrian  sediments  of  British  Columbia,  Alberta,  Idaho, 
and  Montana,  and  in  many  other  parts  of  the  world,  may  be  thus 
explained. 
Eozoic  limestones,  dolomites,  magnesian  limestones,  and  calcareous 
and  magnesian  deposits  generally  were  chemically  deposited  through 
the  medium  of  organic  ammonium  carbonate.  This  alkali  acted  on 
the  primeval  calcium  and  magnesium  salts  of  the  ocean  and  on  the 
calcium  and  magnesium  salts  introduced  to  the  ocean  by  pre-Cam- 
brian rivers.  A  similar  origin  is  suggested  for  the  iron  carbonate 
occurring  in  Eozoic  sedimentary  beds.  It  is  also  suggested  that 
possibly  the  silica  of  the  cherts  and  jaspers  characteristically  asso- 
ciated with  these  carbonates  was  likewise  thrown  out  of  solution 
by  ammonium  carbonate  of  organic  origin.  The  petroleum  and 
natural  gas  emanations  from  Eozoic  sedimentary  rocks  receive  ex- 
planation if  the  fundamental  postulate  of  abundant  Eozoic  marine 
life  be  accepted. 
Coleman,43  in  1907,  argues  for  a  Lower  Huronian  ice  age  on  the 
evidence  of  the  nature  of  the  graywacke  and  conglomerate  in  the 
Lower  Huronian  rocks  of  the  Cobalt  district  and  adjacent  territory. 
Van  Hise,44  in  1908,  outlines  the  development  of  knowledge  of 
the  dual  division  of  the  pre-Cambrian,  emphasizes  the  fact  that  this 
is  based  on  physical  differences  rather  than  on  differences  in  life 
evidences,    and   concludes   that   no    subdivision    is   possible   on    the 
