384  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
intrusive  rock.  By  Shaler  the  lamination  of  the  minerals  in  the  sye- 
nites and  granites,  and  particularly  the  more  distinct  lamination  of  the 
exterior  parts  of  the  exposures,  is  regarded  as  unquestionable  evidence 
of  sedimentary  origin.  Jackson,  among  the  older  geologists,  has  stead- 
ily maintained  the  essentially  igneous  origin  of  the  granites  and  sye- 
nites. Hawes  was  the  first,  however,  to  clearly  study  from  the  modern 
point  of  view  the  granites  aud  granite- gneisses  and  to  show  that  the 
lamination  of  the  latter  is  not  necessarily  an  evidence  of  original  bed- 
ding, and  that  such  a  structure  may  appear  in  an  igneous  rock  as  well 
as  in  a  sedimentary  one.  It  is  only  since  the  recent  work  of  Puinpelly 
and  Emerson  that  it  has  been  generally  appreciated  that  there  are  two 
classes  of  granites  in  New  England.  These  geologists  have  shown  that 
in  western  Massachusetts  is  a  granitoid  gneiss,  which  beyond  all  ques- 
tion antedates  the  Cambrian  rocks  and  has  yielded  debris  to  them. 
While  this  is  the  case,  the  greater  mass  of  the  granites  are  far  later 
in  age,  some  of  them  ranking  in  time  as  late  as  the  Carboniferous. 
Frequently  the  masses  are  so  large  as  to  metamorphose  the  sedimen- 
tary beds  about  them  by  contact  or  dynamic  action,  or  more  probably 
by  both3  producing  the  concentric  schistose  structure  so  early  men- 
tioned by  Percival  and  so  fully  described  by  Emerson. 
By  the  early  advocates  of  metamorphic  granite  a  massive  form  was 
regarded  as  evidence  of  the  completed  process  and  of  the  great  age  of 
the  granite.  In  any  given  granite  the  age  of  which  is  not  known,  from 
the  modern  point  of  view,  its  perfectly  fresh  granular  form,  or  an  even 
lamination  of  mineral  constituents  which  results  from  crystallization 
under  ordinary  circumstances,  in  great  beds  or  masses,  bears  rather 
toward  its  late  formation ;  while  the  contorted  and  foliated  granitoid 
gneisses,  because  of  their  structures,  show  that  they  have  undergone 
repeated  powerful  dynamic  actions,  and  consequently  are  more  likely 
to  be  ancient  rocks. 
The  u  diorite- schists,"  "  ainphibolites,"  "  metamorphic  diorites,"  and 
similar  schistose  rocks  described  even  by  Hawes  as  metamorphic, 
probably  belong  for  the  most  part  with  the  other  greenstones,  which 
have  almost  universally  in  late  years  been  placed  with  the  eruptives. 
These  schistose  phases  are  more  ancient  than  the  massive  forms  or 
have  been  subjected  to  more  intense  metamorphism. 
While  it  was  early  recognized  by  most  geologists  that  slaty  cleavage 
and  foliation  may  cut  across  the  bedding,  it  was  generally  assumed 
that  cleavage  and  stratification  foliation  correspond.  As  early  as  1842 
Percival  so  clearly  saw  the  danger  of  this  course  that  he  states  that  he 
prefers  to  use  the  term  parallel  instead  of  stratified  in  describing  the 
structures  of  the  crystalline  rocks  as  expressing  the  fact  of  the  arrange- 
ment of  the  minerals  without  implying  any  opinion  as  to  the  mode  of 
formation.  Hawes,  in  1878,  states  that  the  granite- gneisses  in  their 
affinities  are  like  the  eruptive  granites,  the  lamination  being  an  induced 
structure  which  may  or  may  not  correspond  with  bedding  in  case  they 
