586  PRE-CAMBRIAN   GEOLOGY   OF   NORTH   AMERICA. 
rocks.    The  order  and  thickness  of  the  crystalline  formations  are  as 
follows,  from  above  downward : 
Section  of  the  crystalline  formations  of  New  Hampshire  and  Vermont. 
Feet. 
Calciferous  mica  schist  and  Coos  group _  12,000 
Kearsarge  group 1,  300 
Rockingham  mica  schist 6,000 
Merrimac  group 4,  300 
Huronian 12,  000 
Hornblende  schist 1,  500 
Montalban 10,  000 
Lake  Winnipiseogee   (Green  Mountain)    gneiss 18,600 
Bethlehem  gneiss —     6,300 
Porphyritic  gneiss 5,  000 
77,  000 
The  various  groups  are  classified  according  to  stratig'raphic  and  not 
iithological  reasons.  Unlike  rocks  are  never  assumed  to  be  identical. 
If  a  hornblende  schist  and  a  clay  slate  dip  toward  each  other  they  are- 
assumed  to  be  of  different  age  and  to  be  separated  by  a  fault.  If  a 
granitic  rock  shows  foliation  it  is  classed  among  the  gneisses.  The 
igneous  rocks  are  devoid  of  marks  of  stratification.  The  Montalban  is 
used  to  cover  pre-Huronian  and  post  or  Upper  Laurentian  rocks. 
Huronian  is  used  for  convenience  to  designate  the  various  schists  of 
chloritic  and  argillitic  aspect  overlying  the  gneisses  and  inferior  to 
the  Cambrian  so  far  as  known.  The  Ascutney  granite  seems  to  have 
been  erupted  from  below  through  one  or  more  vents  and  spread  over 
the  rock  adjacent,  as  is  shown  by  the  fact  that  in  the  valleys  where 
erosion  has  cut  into  the  base  of  the  granite  it  is  discovered  that  schists 
run  under  the  igneous  rock  certainly  for  300  feet.  The  mica  schists 
show  the  presence  of  heat  for  a  distance  of  500  feet  or  more  from  the 
granite.  The  slates  have  been  indurated  so  that  they  ring  like  iron 
when  struck  with  a  hammer.  The  limestones  are  sometimes  calcined 
and  even  glazed.  Veins  enter  both  of  the  rocks  from  points  several 
yards  distant.  The  gneiss  is  not  altered  by  the  contact  line.  It  would 
seem,  therefore,  as  if  we  had  here  examples  of  contact  phenomena, 
and  only  the  later  strata  are  affected,  because  the  gneiss  had  been  al- 
ready made  crystalline  before  the  eruption  of  the  granite. 
Hitchcock  (C.  H.),90  in  1886,  divides  the  older  rocks  of  Vermont 
into  (1)  granite  (Devonian);  (2)  Eozoic  gneiss;  (3)  Potsdam  and 
later  formations.  In  3  are  included  the  Georgia  slates,  the  calciferous 
mica  schist,  etc.  The  Eozoic  gneiss  occurs  in  five  areas  and  is  be- 
lieved to  underlie  the  Potsdam  or  Quebec  group.  At  Wallingford  the 
quartzite  is  superimposed  upon  a  gneiss,  as  shown  by  peculiar  erosion. 
At  Sunderland,  East  Wallingford,  llipton,  and  Bristol,  Vt.,  and 
Clarksburg,  Mass.,  the  fossiliferous  rocks  contain  pebbles  of  a  peculiar 
