584  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
Feet. 
5.  Argillaceous  schists  and  associated  slates,  etc 600 
6.  Quartzite  conglomerate,  with  grits  and  some  argillaceous  schist 
(conglomerate  II) 750 
7.  Carbonaceous  schists  and  shales,  with  argillaceous  schists 500 
8.  Fine  argillaceous  conglomerate  and  grit 500 
5050-5300 
Collie,81  in  1895,  describes  the  geology  of  Conanicut  Island.  The 
oldest  rocks  are  a  series  of  slates  of  unknown  age,  into  which  was 
intruded  a  mass  of  granite,  porphyritic  in  character.  This  complex 
was  exposed  to  weathering  influences  until  a  bed  of  debris  lay  upon 
its  surface.  This  surface  wTas  depressed  beneath  the  sea,  and  upon  it 
was  laid  a  great  series  of  Carboniferous  rocks.  The  complex  wTas, 
therefore,  the  Carboniferous  shore  line.  Into  the  Carboniferous 
rocks  dikes  wTere  intruded,  and  both  were  folded  and  metamorphosed, 
and  have  in  many  places  become  schistose. 
Woodworth,82  in  1899,  describes  the  Algonkian  rocks  in  the  lower 
portion  of  the  Blackstone  Valley  and  west  of  Providence,  near  the 
western  margin  of  the  Narragansett  basin.  Because  of  the  typical 
development  of  the  Algonkian  rocks  along  Blackstone  River  between 
Woonsocket  and  Pawtucket,  they  are  called  the  Blackstone  series. 
This  series  is  divided  into  the  Cumberland  qnartzites;  the  Ashton 
schists,  representing  the  finer  sediments  succeeding  the  deposition  and 
partial  erosion  of  the  Cumberland  quartzites,  and  in  part  probably 
igneous  in  origin;  and  the  Smithfield  limestone,  apparently  of  sedi- 
mentary origin.  As  yet  no  facts  have  been  discovered  to  show 
whether  the  limestones  are  of  the  same  age  or  newer  than  the  Ashton 
schists.  The  rocks  of  the  Blackstone  series  are  separated  and  pene- 
trated by  granitic  intrusions  or  batholiths. 
The  Blackstone  series  is  assigned  to  the  Algonkian  because  of  the 
difference  in  metamorphism  of  the  Blackstone  series  and  the  Lower 
Cambrian  strata,  bearing  Olenellus  fauna,  in  North  Attleboro.  The 
Cambrian  strata  are  little  altered  and  lie  in  close  proximity  to  the 
granite.  Four  miles  west  the  Blackstone  series  is  infolded  with  a 
similar  granite  and  is  much  altered. 
Kemp,83  in  1899,  gives  a  petrographic  account  of  the  pre-Cambrian 
and  later  granites  of  the  Atlantic  coast.  He  finds  a  striking  predomi- 
nance of  biotite  granites  and  gneisses. 
Emerson  and  Perry,84  in  1907,  map  and  describe  the  green  schists 
and  associated  granites  and  porphyries  of  Rhode  Island.  Pre-Cam- 
brian  gneiss  occupies  the  southwestern  part  of  the  area  mapped.  It 
is  probably  the  southward  extension  of  the  Northbriclge  gneiss  ex- 
tending southward  from  Southboro,  Mass.,  and  is  called  the  North- 
bridge  gneiss.     It  is  overlain  unconformably  by  Cambrian  rocks. 
