96  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
foundland  into  Georgia,  and  shows  their  distribution  on  a  sketch 
map.  The  areas  of  these  ancient  volcanic  rocks  now  known  fall 
roughly  in  two  parallel  belts ;  the  eastern  belt  embraces  the  exposures 
of  Newfoundland,  Cape  Breton,  Nova  Scotia,  the  Bay  of  Fundy, 
coast  of  Maine,  Boston  basin,  and  the  central  Carolinas ;  the  western 
belt  crosses  the  Eastern  Townships  and  follows  the  Blue  Ridge 
through  southern  Pennsylvania,  Maryland,  Virginia,  and  North  Car- 
olina to  Georgia.  This  is  largely  a  summary  from  the  work  of  other 
writers,  supplemented  by  field  observations  in  the  Piedmont  area. 
For  the  most  part  age  is  not  discussed,  although  the  pre-Cambrian 
age  of  the  South  Mountain  volcanic  rocks  is  mentioned. 
Brooks  (W.  K.)>27  in  1894,  discusses  the  origin  of  the  oldest  fossils 
and  explains  the  lack  of  fossils  in  the  pre-Cambrian  and  their  sudden 
appearance  in  the  Cambrian,  as  follows: 
1.  The  primitive  fauna  of  the  sea  bottom  was  entirely  animal,  with- 
out plants,  and  it  at  first  depended  directly  upon  the  pelagic  food 
supply. 
2.  It  was  established  around  elevated  areas  in  water  deep  enough  to 
be  beyond  the  influence  of  the  shore. 
3.  The  great  groups  of  animals  were  rapidly  established  from 
pelagic  ancestors. 
4.  The  animals  of  the  bottom  rapidly  increased  in  size  and  hard 
parts  were  quickly  acquired. 
5.  The  bottom  fauna  soon  produced  progressive  development 
among  pelagic  animals. 
6.  After  the  establishment  of  the  fauna  of  the  bottom,  elaboration 
and  differentiation  among  the  representatives  of  each  primitive  type 
soon  set  in  and  led  to  the  extinction  of  connecting  forms. 
Thus  is  explained  the  high  degree  of  differentiation  and  develop- 
ment shown  by  the  early  Cambrian  fauna. 
Van  Hise,28  in  1896,  discusses  principles  of  North  American  pre- 
Cambrian  geology.  The  first  half  of  the  volume  treats  of  the  prin- 
ciples of  structural  geology  particularly  applicable  to  pre-Cambrian 
study.  On  a  structural  basis  the  earth's  lithosphere  is  assumed  to  be 
made  up  of  a  zone  of  fracture  at  the  surface,  grading  below  into  a 
zone  of  rock  flowage,  and  attempts  are  made  on  several  hypotheses 
to  determine  the  depth  below  the  surface  to  which  rock  flowage  must 
begin  because  of  the  conditions  of  pressure  there,  and  it  is  concluded 
that  this  depth  is  seldom  greater  than  10,000  meters.  The  structural 
phenomena  of  the  zone  of  rock  flowage  are  discussed  under  the  heads 
of  folding  and  cleavage,  and  those  of  the  zone  of  rock  fracture  under 
the  heads  of  joints,  faults,  fissility,  etc.  Other  subjects  discussed 
are  methods  of  field  observation  in  closely  folded  districts,  criteria 
for  determining  unconformities,  and  some  of  the  elementary  princi- 
ples of  the  metamorphism  of  rocks. 
