302  PRE-CAM  BRIAN    GEOLOGY   OF    NORTH    AMERICA. 
other  places.  Acidic  flows  are  superimposed  upon  basic  flows,  flows  of 
intermediate  acidity  immediately  overlie  acidic  flows,  flows  of  inter- 
mediate acidity  overlie  porphyritic  conglomerates,  flows  of  interme- 
date  acidity  are  superimposed  upon  basic  flows,  basic  rocks  are  inter- 
sected by  acidic  rocks,  basic  flows  overlie  acidic  rocks,  basic  flows  over- 
lie those  of  intermediate  acidity,  and  acidic  rocks  are  intersected  by 
basic  rocks.  There  is  a  complete  absence  from  the  series  of  anything 
like  volcanic  ash.  The  detrital  rocks  of  the  series  are  composed  of 
fragments  broken  for  the  most  part  from  the  acidic  rocks  of  the 
series — that  is,  such  material  as  porphyry,  both  nonquartziferous 
and  quartziferous,  felsite,  augite  syenite,  granitell,  and  granite;  but 
there  are  also  often  found  pebbles  of  the  basic  rocks,  and  in  some 
cases  particles  of  gneiss  and  granite  from  the  underlying  series.  This 
is  thought  to  be  due  to  the  fact  that  such  viscous  material  would 
solidify  into  more  or  less  bulky,  erect  masses  of  relatively  small  area, 
and  thus  be  most  favorably  situated  for  degradation.  Between  the 
several  kinds  of  original  rocks  there  are  no  sharp  lines,  but  a  continu- 
ous series  of  kinds  from  the  most  basic  to  the  most  acidic. 
The  lithology  of  the  different  members  of  the  series  is  given  in  de- 
tail. The  basic  original  rocks  include  granular,  porphyritic,  and 
glassy  kinds,  the  most  abundant  of  which  are  gabbro,  diabase,  mela- 
phyre,  and  porphyrite.  The  acidic  original  rocks  include  quartzless 
porphyry,  quartziferous  porphyry,  and  felsite,  augite  syenite,  granite 
porphyry,  and  granite.  Here  are  included  the  so-called  "  jaspers," 
which  have  been  regarded  by  many  as  metamorphosed  sedimentary 
rocks.  The  basic  crj^stalline  rocks  make  up  the  greater  part  of  the 
thickness  of  the  series,  the  beds  varying  from  a  few  feet  to  several 
hundred  feet  in  thickness.  Each  of  these  beds  often  has  a  twofold 
division,  an  upper  amygdaloidal  portion,  and  a  lower  compact  por- 
tion, which,  however,  grade  into  each  other.  The  amygdaloids  not 
infrequently  resemble  beds  of  sedimentary  origin,  but  they  never 
show  any  trace  of  fragmental  character,  and  the  stratiform  condition 
is  seen  to  be  due  to  a  succession  of  thin  flows  and  two  fluidal  struc- 
tures. Laterally  the  beds  are  not  of  indefinite  extent,  and  are  far 
less  extensive  than  sedimentary  beds  of  the  same  thickness.  It  is  gen- 
erally, however,  difficult  to  prove  the  continuity  or  noncontinuity  of 
a  single  flow  over  a  great  distance,  but  on  the  Minnesota  shore  in- 
dividual layers  were  traced  with  certainty  for  10  or  15  miles,  while 
other  beds  almost  certainly  have  an  extent  of  nearly  30  miles,  and 
groups  of  layers  of  allied  characters  are  recognizable  over  much 
longer  stretches.  The  more  massive,  thicker  beds  generally  occur  in 
the  lower  part  of  the  series.  Numerous  dikes  cut  the  basic  rocks. 
These  are  generally  small,  commonly  not  more  than  10  feet  in  width, 
but  in  the  immediately  underlying  series  on  the  north  shore,  the 
Lower  Copper-bearing  or  Animikie  group,  are  dikes  of  great  magni- 
