GENERAL  ACCOUNT    OF    THE    PEE-CAMBRIAN    ROCKS.  49 
surface  >eneath  the  waters  of  Torridonian  time.  Between  Lake  Maree  and  Loch 
Broom  it  is  possible  to  determine  the  orientation  of  these  buried  valleys  and  to 
prove  that  some  of  the  hills  exceeded  2,000  feet  in  height. 
In  the  memoir  the  Lewisian  system  is  often  referred  to  as  the 
u  Fundamental  complex  "  in  exactly  the  same  sense  as  we  have  used 
the  term  in  the  Lake  Superior  region  for  the  pre-Algonkian  system. 
Indeed,  the  Scotland  "  Fundamental  complex  "  has  all  the  character- 
istics of  that  of  the  Lake  Superior  region.  After  years  of  work  by 
several  men  much  the  larger  part  of  the  Lewisian  is  mapped  as 
"  undifferentiated  " — that  is,  it  has  not  been  possible  in  western  Scot- 
land, with  the  splendid  exposures  of  the  region  and  with  6  inches  to 
the  mile  Ordnance  maps  of  the  highest  quality,  to  separate  the  funda- 
mental complex  into  formations,  except  for  small  areas.  Probably 
nowhere  else  in  the  world  are  the  opportunities  better  to  make  such 
separation,  if  this  were  possible,  and  certainly  no  other  geological 
survey  has  ever  spent  more  than  a  small  fraction  of  the  time  given 
by  the  Scotland  Survey  to  a  small  area  of  the  fundamental  complex. 
The  petrographic  descriptions  sIioav  that  the  Lewisian  is  domi- 
nantly  composed  of  igneous  rocks,  of  which  the  granitic  type  is  most 
abundant,  and  that  the  sediments  are  extremely  subordinate.  The 
latter  rocks  include  "  mica  schists,  graphitic  schists,  quartz  schists, 
siliceous  granulites,  limestones,  dolomites,  and  cipolins."  a 
The  relations  of  the  sediments  of  the  Lewisian  to  the  igneous  series 
have  not  been  entirely  worked  out.     Home  says : b 
There  is  no  clear  evidence  that  these  types  are  intrusive  in  the  former,  but 
in  certain  places  the  two  are  so  intimately  associated  as  to  suggest  that  the 
rocks  of  igneous  origin  may  have  been  injected  into  those  of  sedimentary  origin. 
On  the  other  hand,  there  is  undoubted  proof  that,  north  of  Lake  Maree,  the 
altered  sediments  rest  on  a  platform  of  gneiss  and  are  locally  overlain  by  gneiss 
with  basic  dikes,  the  superposition  of  the  gneiss  on  the  sediments  being  there 
due  to  folding  and  thrusting. 
In  the  strongest  possible  contrast  with  the  Lewisian  is  the  Tor- 
ridonian. Here  ordinary  stratigraphic  methods  apply,  and  the  system 
has  been  divided  into  three  formations. 
It  is  clear  that  under  the  general  classification  advocated  in  this 
report  the  Torridonian  is  Algonkian  and  the  Lewisian  is  Archean. 
While  the  foldings,  faultings,  intrusions,  and  metamorphisms  of  the 
Archean  have  been  so  extreme  and  the  relations  of  the  different  rocks 
so  intricate  that  upon  the  general  geological  maps  there  lias  been  no 
attempt  to  subdivide  the  fundamental  complex,  the  descriptions 
show  that  in  the  central  district  basic  rocks  are  developed  in  great 
force  with  subordinate  amounts  of  ultra-basic  rocks,  and  that  in  the 
a  Op.   eit,   p.   75.  b  Op.  cit.,  p.  4, 
55721— Bull.  360—09 4 
