380  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86 
ably  belong  to  the  Hudson  river.  The  structure  of  the  range  is  a  com- 
pound synclinal.  Mount  Washington  is  a  synclinal  of  the  same  kind, 
which  dies  out  to  the  south  with  a  multiplication  of  small  subordinate 
flexures. 
Hitchcock  (0.  H.),83  in  1884,  describes  a  number  of  geological 
sections  across  New  Hampshire  and  Vermont,  and  correlates  the  rocks. 
The  order  and  thickness  of  the  crystalline  formations  from  above  down- 
ward are  as  follows: 
Feet. 
Calciferous  mica-schist  and  Coos  group 12,  000 
Kearsar^e  group 1 ,  300 
Rockingham  mica-schist 6,  000 
Merrimac  group ^ ,  300 
Huronian 12,  000 
Hornblende-schist 1 ,  f>00 
Montalban 10,  000 
Lake  Winnipiseogee  (Green  mountain)  gneiss 18,  000 
Bethlehem  gneiss 0,  300. 
Porphyritic  gneiss 5,  000 
Total  thickness ! 77, 000 
The  various  groups  are  classified  according  to  strati  graphical  and  not 
lithological  reasons.  Unlike  rocks  are  never  assumed  to  be  identical. 
If  a  hornblende-schist  and  clay-slate  dip  toward  each  other  they  are 
assumed  to  be  of  different  age  and  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  Avhere  erosion  has  cut  into 
the  base  of  the  granite  it  is  discovered  that  schists  run  under  the  igne- 
ous 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  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  ail 
affected,  because  the  gneiss  had  been  already  made  crystalline  before 
the  eruption  of  the  granite. 
Hitchcock,  (C.  H.),84  in  1886,  divides  the  older  rocks  of  Vermont  into 
(1)  granite  (Devonian);  (2)  Eozoic  gneiss;  (3)  Potsdam  and  later  for-; 
mations.  In  (3)  are  included  the  Georgia  slates,  the  calciferous  mica- 
schist,  etc.  The  Eozoic  gneiss  occurs  in  five  areas  and  is  believed  to 
underlie  the  Potsdam  or  Quebec  group.    At  Wallingford  the  quartzi  te  is 
