574  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
Burbank,56  in  1876,  lithologically  divides  the  formations  of  the 
Nashua  Valley  into  (1)  argillite,  (2)  mica  slate  and  quartzite,  and 
(3)  granite  and  granitoid  gneiss.  With  the  argillite  and  mica  slate 
are  small  beds  of  conglomerate,  and  inclosed  in  the  gneiss  are  nodular 
masses  of  crystalline  magnesian  limestone.  In  the  slates  at  Harvard 
and  Bolton  occurs  conglomerate  in  which  the  pebbles  in  many  cases 
are  flattened,  bent,  and  even  drawn  out  into  layers,  giving  an  agate- 
like structure  to  the  rock.  The  principal  conglomerate  beds  lie  be- 
tween hills  of  granite  on  the  west  and  north  and  mica  slate  and  gneiss 
on  the  south,  yet  not  a  pebble  of  granite  or  gneiss  has  been  discovered. 
As  to  the  age  of  the  Nashua  rocks  no  positive  opinion  is  offered,  but 
the  author  is  inclined  to  regard  them  as  belonging  to  a  distinct  system 
older  than  the  Wachusett  gneiss.  The  mica  slate  appears  to  be  inter- 
stratified  with  and  to  overlie  the  gneiss,  and  the  argillite  beds  appear 
to  be  for  the  most  part  conformable  with  the  mica  slate. 
Dana,57  in  1877,  maintains  that  the  garnetiferous  mica  slate,  stau- 
rolitic  slate,  mica  schist,  gneiss,  and  quartzite  of  Bernardston  are 
Helderberg,  on  the  ground  that  in  these  rocks  are  found  fossils  indi- 
cating this. 
Wadsworth,58  in  1879,  describes  the  felsites  of  Marblehead  Neck  as 
altered  rhyolites  which  show  characteristic  fluidal  structure.  These 
felsites  are  not  stratified,  and  are  younger  than  the  granite  on  the 
neck,  as  dikes  of  felsite  are  seen  cutting  it.  There  is  no  passage  of  the 
conglomerate  into  the  felsite  in  this  locality. 
Crosby,59  in  1880,  divides  the  Azoic  formations  of  eastern  Massa- 
chusetts into  Naugus  Head,  Huronian,  and  Montalban,  these  terms 
being  used  on  account  of  a  lithological  and  stratigraphical  resem- 
blance which  they  bear  to  the  Azoic  divisions  of  other  regions.  The 
Naugus  Head  is  provisionally  regarded  as  equivalent  to  Hunt's  No- 
rian,  and  the  Huronian  older  than  the  Montalban.  The  entire  Nau- 
gus Head  formation  seems  to  have  been  plastic,  and  the  extravasation 
has  been  so  extensive  that  the  character  of  the  rocks  changes  at  nearly 
every  rod  to  varieties  regarded  as  composed  of  metamorphosed  strati- 
fied rocks.  The  Naugus  Head  series  has  been  extensively  extravasated 
through  the  superjacent  Huronian  formation,  but  is  penetrated  by 
nothing  foreign  to  itself,  the  Huronian  granite  being  never  found  to 
cut  the  Naugus  Head  series,  from  which  the  conclusion  is  formed  that 
the  Naugus  Head  is  older  than  the  Huronian.  In  fact,  the  Huronian 
granite  pierces  every  rock  in  the  region  save  the  Naugus  Head  series 
and  the  newer  noncrystallines,  so  that  it  can  not  be  doubted  that  it 
overlies  this  ancient  terrane.  The  latter  must  be  regarded  as  the 
lowest,  and  hence  the  oldest,  member  of  the  succession. 
The  Huronian  is  principally  composed  of  granite,  petrosilex,  dio- 
rite,  hornblendic  gneiss,  and  limestone,  this  being  the  order  of  age 
from  the  base  upward.    The  petrosilex  is  the  most  characteristic  rock. 
