450  PRE-CAMBEIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
of  structure  is  the  development  of  conoidal  shapes  formed  of  concentric 
layers  varying  in  thickness  from  a  few  inches  to  100  feet.  This  structure, 
so  far  as  observed,  is  strictly  confined  to  the  hornblende-bearing  gran- 
ites, and  never  makes  its  appearance  in  those  of  the  first  and  second 
types.  Although  instances  of  each  type  of  granitic  rock  are  found  uncon- 
formably  underlying  the  low  members  of  the  Paleozoic  series,  this  is 
not  the  case  with  each  outcrop ;  many  granitic  masses  are  found  uncon- 
formably  underlying  Mesozoic  or  even  Tertiary  volcanic  rocks.  But 
there  is  absolutely  no  evidence  whatever  in  favor  of  the  belief  of 
granitic  extrusions  later  than  the  Archean  age.  Although  Whitney 
has  found  intrusions  of  granite  in  sedimentary  strata  other  than  the 
Archean  crystalline  schists,  any  attempt  to  correlate  age  by  penolog- 
ical features  alone  is  dangerous,  as  may  be  shown  by  the  fact  that  the 
Jurassic  granite  of  California  and  the  granite  of  the  Cottonwood  region 
on  the  Wasatch,  which  is  unmistakably  Archean,  are  positively  iden- 
tical down  to  the  minutest  microscopical  peculiarity. 
In  the  crystalline  schists  and  gneisses  are  found  identically  the  same 
minerals  which  characterize  the  granites.  In  the  schists  the  charac- 
teristic feature  is  the  parallel  bedded  arrangement.  Granite  possesses 
the  same  minerals ;  the  sole  difference  seems  to  be  that  granite  is  often 
demonstrably  a  plastic  intrusion  and  possesses  no  parallel  arrangement 
of  minerals.  The  geognostic  position  of  the  schists  is  exactly  like  the 
other  strata  which  were  deposited  horizontally  and  afterwards  dis- 
turbed. On  the  other  hand,  granite  in  an  immense  majority  of  cases 
is  found  exposed  in  the  hearts  of  the  mountain  ranges.  It  is  only 
when  we  can  observe  granite  in  direct  connection  with  the  strata  into 
which  it  has  intruded  or  out  of  which  it  has  been  made  that  the  true 
relations  can  be  seen;  and  it  is  safe  to  say  that  wherever  these  inti- 
mate relations  are  observable,  the  granite  occupies  a  region  which  has 
been  subjected  to  horizontal  or  circumferential  pressure.  The  frequent 
phenomena  of  the  under-dip  of  the  strata  flanking  a  granite  mass,  as  in 
the  great  granite  body  of  the  Sierra  Nevada,  are  prominent  instances 
of  the  intimate  relation  spoken  of.  If  in  such  cases  an  unconformable 
overlying  and  unaltered  series  were  to  cover  all  but  the  summits  of 
the  granite  hills,  the  granite  would  appear  simply  as  an  unconform- 
able underlying  body  whose  genetic  relations  are  absolutely  unknown. 
Into  this  category  a  vast  number  of  granite  exposures  of  the  Cordilleras 
have  to  be  placed. 
It  is  an  invariable  law,  then,  that  where  the  genetic  relations  are 
clearly  perceived,  eruptive  granite  is  always  found  in  connection  with 
very  great  horizontal  pressure  and  consequent  disturbance.  Suppose^ 
now,  a  deep-lying  series  of  varied  sedimentary  beds,  covered  by.a  suffi- 
cient superimposed  mass  to  exert  a  pressure  powerful  enough  to  sink 
them  to  the  necessary  thermal  horizon  for  the  induction  of  crystalliza- 
tion in  the  material  of  the  beds.  As  long  as  the  attitude  of  these  beds 
was  undisturbed  by  horizontal  compression,  the  result  would  be  a 
