366  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  Ibull.86. 
less  than  1,000  feet.  The  principal  masses  are  coarsely  crystalline 
magnesian  limestones,  homogeneous  in  texture,  and  showing  no  traces 
of  stratification.  The  various  silicates  occur  attached  to  or  near  the 
inclosing  walls  of  the  cavities. 
Dodge,48  in  1875,  divides  the  rocks  of  eastern  Massachusetts  into 
two   groups,  the  crystallines  and  the  more  clearly   stratified  rocks 
among  them.     In  the  crystallines  are  placed  the  syenite  and  greenstone.  I 
These  rocks  have  a  dip  to  the  west  or  northwest,  and  they  uncou- 
formably  underlie  strata   holding  Paradoxides.     For  the  most  part 
metamorphism  has  been  so  complete  that  the  rocks  have  entirely  lost 
their  original  character.    While  eruptive  rocks  have  often  an  appear- 
ance of  schistose  structure,  in  metamorphic  syenites  and  diorites,  on  I 
the  other  hand   the  original    stratification  is  often  completely  lost. ! 
Throughout  the  crystalline  area  there  are  immense  masses  of  horn- 
blende  rock,  diabase,  and  diorite,  usually  crypto- crystalline,  in  which 
no  indication  of  sedimentary  origin  can  be  traced.     The  syenites  con-l 
sist  of  quartz,  feldspar,  with  little  or  no  hornblende.     The  porphyry  I 
probably  belongs  with  the  crystalline  group,  and  pebbles  of  it  are  j 
abundant  in  the  Brighton  conglomerates.     Perhaps  some  of  the  slates  I 
are  so  altered  in  this  region  as  to  resemble  real  porphyry. 
Among  the  second  division  are  siliceous  slates  and  breccias.  The  | 
siliceous  slates  are  often  much  contorted.  At  Arlington  they  pass  j 
through  fine  grits  and  coarse  syenites  by  various  stages.  The  crystal- 
lines occupy  distinct  bands,  separated  by  more  recent  rocks  collected  I 
in  the  area  between  them.  These  more  recent  rocks  are  shown  to  bel 
such  by  their  position  in  relation  to  the  underlying  crystallines,  as  well  J 
as  by  the  fact  that  they  are  composed  of  detritus  of  the  latter.  In  | 
places  they  are  fossiliferous,  and  at  Braiutree  contain  the  Paradoxides! 
fauna.  These  stratified  rocks  are  in  part  slates  and  in  part  conglonJ  j 
erates,  the  former  appearing  to  occupy  the  inferior  position.  The  con- 
glomerates are  well  developed  in  the  vicinity  of  Newport  and  Newberry.  1 
Cutting  the  slates  and  conglomerates  are  rocks  which  have  been  called  j 
eruptives,  but  so  close  is  the  resemblance  in  chemical  and  mineral  com-  j 
position  and  in  appearance  to  the  more  fusible  portions  of  the  erys-  I 
tallines  that  it  seems  almost  unreasonable  to  doubt  that  the  former  I 
were  derived  from  many  deep  lying  masses  of  the  latter. 
Crosby,49  in  1876,  describes  and  maps  the  Eozoic  rocks  of  Massachu- 1 
setts.     They  are  divided  into  Norian,  Huronian,  and  Montalban  onl 
lithological  and  chronological  grounds.     The  lithological  characters  of- 
the  divisions  are  as  unlike  as  the  fauna  of  any  two  successive  geolog-  * 
ical  formations.     The  Norian  is  found  in  two  areas  in  Massachusetts;  J 
that  including  the  city  of  Salem  and  adjacent  region,  and  that  which 
includes  the  seaward  end  of  large  Nahant.     The  Norian  rocks,  com- 
posed chiefly  of  feldspar,  hornblende,  and  pyroxene,  are  sometimes 
stratified  and  in  other  places  massive.    The  Huronian  rocks  occur 
over  a  wide  area,  having  an  extreme  length  of  65  miles  and  an  extreme 
