a.n ;  hi*e .]  EASTERN  UNITED  STATES.  367 
Badth  of  aboul  40  miles.  The  Huronian  comprises  areas  marked  on 
Lie  geological  map  of  Hitchcock  as  syenite,  porphyry,  and  hornblende- 
late.  The  rocks  here  included  are  treated  under  hornblendic  granite, 
Kite,  diorite,  stratified  rocks,  arid  limestones,  all  of  which  are  regarded 
I  metamorphosed  sedimentary  beds.  The  Montalban  includes  granite, 
rhicli  comprises  exotic,  indigenous  and  endogenous  forms,  gneiss,  mica- 
late,  argillite,  and  limestone.  Most  of  the  slates  in  the  vicinity  of 
>oston  are  regarded  as  of  Primordial  age,  although  fossils  hav£  only 
een  found  at  Braintree.  The  argillite  of  Kents  island  and  the  meta- 
lorphic  slate  of  Newport  are  also  regarded  as  Paleozoic. 
Burbank,50  in  1876,  lithologically  divides  the  formations  of  the 
Nashua  valley  into  (1)  argillite;  (2)  mica-slate  and  quartzite;  (3) 
raiiite  and  granitoid  gneiss.  With  the  argillite  and  mica-slate  are 
mall  beds  of  conglomerate,  and  inclosed  in  the  gneiss  are  nodular 
lasses  of  crystalline  magnesian  limestone.  In  the  slates  at  Harvard 
ud  Bolton  occurs  conglomerate  in  which  the  pebbles  in  many  cases 
re  flattened,  bent,  and  even  drawn  out  into  layers,  giving  an  agate  - 
ke  structure  to  the  rock.  The  principal  conglomerate  beds  lie  between 
ills  of  granite  on  the  west  and  north  and  mica-slate  and  gneiss  on  the 
mth,  yet  not  a  pebble  of  granite  or  gneiss  has  been  discovered.  As 
)  the  age  of  the  Nashua  rocks  no  positive  opinion  is  offered,  but  the 
uthor  is  inclined  to  regard  them  as  belonging  to  a  distinct  system 
kler  than  the  Wachusett  gneiss.  The  mica-slate  appears  to  be  inter - 
}ratilied  with  and  overlie  the  gneiss,  and  the  argillite  beds  appear  to 
e  for  the  most  part  conformable  with  the  mica-slate. 
Dana,51  in  1877,  maintains  that  the  garnetiferous  mica  slate,  stauro- 
tic  slate,  mica-schist,  gneiss,  and  quartzite  of  Bernardston  are  Helder- 
erg,  on  the  ground  that  in  these  rocks  are  found  fossils  indicating  this. 
Wadswobth,52  in  1879,  describes  the  felsites  of  Marblehead  neck  as 
tered  rhyolites,  which  show  characteristic  fluidal  structure.  These 
lsites  are  not  stratified,  and  are  younger  than  the  granite  on  the  neck, 
5  dikes  of  felsite  are  seen  cutting  it.  There  is  no  passage  of  the  con- 
iomerate  into  the  felsite  in  this  locality. 
Crosby,53  in  1880,  divides  the  Azoic  formations  of  eastern  Massa- 
chusetts into  Nangus  head,  Huronian,  and  Montalban,  these  terms 
ing  used  on  account  of  a  lithological  and  strati  graphical  resemblance 
hich  they  bear  to  the  Azoic  divisions  of  other  regions.  The  Naugus 
ad  is  provisionally  regarded  as  equivalent  to  Hunt's  Norian  (although 
contains  no  labradorite),  as  older  than  the  Huronian,  and  the  Hiiro 
an  older  than  the  Montalban.  The  entire  Naugus  head  formation 
jlnis  to  have  been  plastic,  and  the  extravasation  has  been  so  exten- 
ve  that  the  character  of  the  rocks  changes  at  nearly  every  rod  to  varic- 
es regarded  as  composed  of  metamorphosed  stratified  rocks.  The 
augus  head  series  has  been  extensively  extravasated  through  the  super- 
cent  Huronian  formation,  but  is  x>enetrated  by  nothing  foreign  to  itself, 
ie  Huronian  granite  being  never  found  to  cut  the  Naugus  head  series, 
