PIEDMONT  PLATEAU  AND  PORTIONS  OF  THE  APPALACHIANS.       7 11 
with  igneous  granitic  rocks  rather  than  with  the  sedimentaries.  Em- 
mons also  reached  the  same  conclusion  for  the  South  which  Emerson 
demonstrated  many  years  later  for  the  New  England  States  thai 
there  are  two  granites,  one  of  which  is  more  ancient  than  the  elastics 
and  the  other  intrusives  within  these.  According  to  Emmons  the 
ancient  form  is  predominant,  but  occasionally  granite  has  been  pro- 
jected through  fissures,  like  other  intrusive  rocks. 
The  great  discovery  that  regularly  laminated  rocks  are  produced  by 
the  metamorphism  of  eruptive  rocks  as  well  as  from  sedimentary 
rocks  naturally  carried  the  discoverer  too  far  in  the  application  of 
the  principle.  Emmons  included  in  the  metamorphic  igneous  rocks 
many  mica  schists,  talcose  slates,  and  limestones,  for  which  lie  gave 
no  evidence  whatever.  Lieber's  discrimination  between  the  metamor- 
phic-igneous  and  metamorphic-sedimentary  rocks  was  much  more 
satisfactory.  But  Emmons's  general  statements  as  to  the  small  value4 
of  lamination  alone  in  rocks  as  evidence  of  origin,  and  his  method 
of  stratigraphic  work  in  the  crystalline  rocks,  can  hardly  be  improved 
upon  at  the  present  day.  Says  this  writer:  Rocks  of  igneous  origin 
are  often  massive,  but  also  are  frequently  laminated,  and  laminated 
rocks  are  frequently  called  stratified,  but  this  latter  term  should  be 
restricted  to  the  sedimentary  rocks.  The  metamorphic;  rocks  are 
excluded  from  the  sedimentary  classification  because  all  rocks  may 
become  metamorphic,  and  a  stratum  metamorphic  in  one  locality 
may  not  be  metamorphic  in  another.  The  highest  proof  of  the  age 
of  rocks  is  the  order  of  superposition.  When  this  method  can  be 
applied  it  is  paramount,  but  paleontology  may  be  used  subject  to 
projDer  principles. 
The  "Primitive"  rocks  from  Emmons's  point  of  view  are  all 
igneous;  with  aqueous  rocks  begins  the  "Azoic,"  the  oldest  sedimen- 
taries, and  above  the  "Azoic"  are  rocks  which  in  the  past  have  been 
regarded  as  azoic  but  are  found  to  be  fossiliferous ;  that  is,  they  con- 
stitute the  "Taconic"  system.  We  have  here  a  definite  theory  as  to 
the  order  of  development  of  the  earth,  the  "Primitive"  rocks  being 
wholly  pyrocrystalline,  the  "Azoic"  stratified  rocks  being  earlier 
than  the  dawn  of  life,  and  the  "Taconic"  rocks  being  the  fossilifer- 
ous rocks  earlier  than  the  Potsdam. 
NOTES. 
'  A  sketch  of  the  geology  of  the  country  near  Easton,  Pa.,  with  a  catalogue 
of  the  minerals  and  a  map,  by  John  Finch.  Am.  Jour.  Sci.,  Isi  ser.,  vol.  8, 
1824,  ]»p.  236-240. 
8 On  the  geology  and  mineralogy  of  the  country  near  West  Chester,  Pa.,  by 
J.  Finch.     Idem,  vol.   1  I.   1828,  pp.   15   18. 
"The  geology  of  Pennsylvania,  by  Henry  Darwin  Rogers,  2  vols..  ;,s.;.  ion; 
pp.,  atlas  of  two  maps,  Philadelphia,  1858.  See  also  Third  Ann.  Rept.  Geol. 
Survey  Pennsylvania,  Harrisburg,  L839,  pp.  118.     Fourth  Ann.  Rept.  Geol.  Sur- 
