GENERAL  ACCOUNT   OF    THE   PRE-CAMBRIAN    ROCKS.  35 
ing  none  for  the  animals;  that  the  great  addition  of  lime  and 
magnesia,  giving  excess  over  that  deposited  by  the  ammonium  car- 
bonate, was  due  to  orogenic  movements,  followed  by  base-leveling 
near  the  close  of  the  pre-Cambrian.  Daly  also  concludes  from  com- 
pilation of  available  analyses  of  pre-Cambrian  carbonate  formations 
that  magnesia  was  relatively  more  abundant  in  the  pre-Cambian  than 
it  has  been  later.  Biologists  now  emphasize  the  deleterious  effects  of 
magnesia  upon  life.  It  is  possible  that  these  facts  are  to  be  correlated. 
Lane"  has  argued  that  the  pre-Cambrian  ocean  was  of  different  com- 
position from  that  of  to-day,  and  that  this  difference  in  composition 
had  an  effect  on  the  development  and  preservation  of  life. 
Whatever  the  real  significance  of  the  contrast  in  fossil  content  be- 
tween Cambrian  and  pre-Cambrian  rocks,  it  stands  as  a  fact  Avhich 
must  be  clearly  recognized  and  which  seems  to  prevent  the  certain 
classification  of  nonfossiliferous  continuous  downward  extensions  of 
the  Olenellus  horizon  with  the  Cambrian  or  Paleozoic. 
LENGTH  OF  ALGONKIAN   TIME. 
The  highly  developed  life  records  of  the  Cambrian,  together  with 
the  thickness  of  the  pre-Cambrian  and  the  existence  of  several  im- 
portant unconformities  between  its  members,  indicate  an  enormous 
lapse  of  time  for  the  Algonkian.  If  time  can  be  measured  by  thick- 
ness of  sediments,  the  Algonkian  was  perhaps  as  long  as  or  longer 
than  all  Paleozoic  time,  and  Algonkian  and  Archean  time  together 
may  have  been  longer  than  all  post-Algonkian  time.  Indeed,  if  the 
amount  of  life  development  be  the  standard  by  which  to  measure  time, 
the  appearance  of  the  Cambrian  fauna  at  the  base  of  the  Paleozoic 
may  be  a  comparatively  modern  event. 
DEFORMATION  AND  METAMORPHISM  OF  THE  ALGONKIAN. 
Algonkian  rocks  have,  on  the  whole,  suffered  more  deformation 
and  metamorphism  than  younger  rocks.  Deformation  has  expressed 
itself  in  folding,  faulting,  jointing,  and  irregular  fracturing.  In 
conjunction  with  these  structures  metamorphism  and  cleavage  have 
so  frequently  developed  as  to  be  common  features.  In  the  amount  of 
structural  deformation  and  attendant  phenomena  there4  is  great  dif 
ference  between  the  members  of  the  Algonkian  system,  even  in  the 
same  district,  and  a  vast  difference  in  different  geological  provinces. 
On  the  whole,  the  phenomena  of  deformation  and  metamorphism  arc 
greater  than  in  the  Paleozoic.  But  the  difference  between  the  Algon 
kian  and  the  Paleozoic  in  these  respects  is  no  greater  than  the  dif- 
ference between  the  Paleozoic  and  the  Mesozoic.  The  contrasts  in 
deformation  and  metamorphism  between  the  Algonkian  ami  the  Pale- 
a  Lane,  A.  C,  The  early  surroundings  of  life  :  Science,  vol.  20,  1907,  pp.  120-143. 
