520  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
viceable  in  discovering  an  unconformity ,  for  the  sets  of  faults  or  joints 
produced  in  the  older  series  before  the  newer  series  was  formed  are  not 
found  in  the  latter.  When  there  have  been  later  orographic  movements 
which  have  produced  faults  or  joints  in  both  series  the  unconformity 
will  be  shown  by  the  presence  in  the  older  series  of  two  sets  of  joints 
or  faults  in  different  directions,  provided  the  directions  of  thrust  were 
different,  while  the  newer  series  will  be  affected  by  joints  or  faults  only 
in  a  single  direction.  If  the  jointing  or  faulting  is  in  the  same  direc- 
tion in  both  the  newer  and  older  movements  they  will  not  be  of  much 
service  in  detecting  an  unconformity,  the  only  difference  being  their 
greater  frequency  in  the  older  series. 
When  one  of  the  orographic  movements  has  resulted  in  folding  and 
the  other  in  faulting  or  jointing,  the  combination  of  phenomena  are  as 
easily  used  to  detect  an  unconformity  as  when  effects  of  the  same  kind 
are  produced  by  both  movements. 
(3)  Discordance  of  the  bedding  of  an  unfoliated  series  with  the 
cleavage  or  foliation  of  an  adjacent  series  may  be  taken  as  evi- 
dence of  unconformity,  if  the  former  is  such  that  it  would  take  on 
cleavage  or  foliation  as  readily  as  the  latter;  for  whatever  the  origin 
of  the  altered  series  the  development  of  cleavage  or  foliation,  which 
must  have  developed  before  the  new  series  was  deposited,  required 
much  time.  An  unconformity  could  not  be  inferred  from  the  fact 
that  a  heavy  formation  of  quartzite  or  of  limestone  cuts  across  the 
cleavage  or  foliation  of  an  argillite  or  mica-schist,  for  clayey  rocks 
very  much  more  readily  take  on  secondary  structures.  In  the  same 
series  it  often  happens  that  more  massive  beds  escape  foliation,  which 
may  be  prominently  developed  in  other  members.  But  if  a  forma- 
tion with  slaty  or  schistose  structure  is  overlain  by  another  forma- 
tion without  secondary  structure  which  from  its  composition  is  as 
likely  to  take  on  foliation  as  the  underlying  formation,  a  discordance, 
while  not  demonstrated,  is  a  probability  for  which  other  evidence 
should  be  sought. 
(4)  Eruptive  rocks  are  often  an  important  guide  in  determining 
structural  discordances.  These  are  valuable  when  the  older  series 
has  passed  through  an  epoch  of  eruptive  activity  before  the  newer 
series  was  deposited.  In  such  cases  bosses,  contemporaneous  or  in- 
trusive beds,  volcanic  fragmental  material  or  dikes  may  occur  in  the 
older  series,  which  nowhere  are  associated  with  the  newer.  It  is  pos- 
sible, of  course,  that  eruptives  may  penetrate  the  inferior  members 
of  a  series  and  never  reach  the  higher  formations ;  but  if  it  is  found 
that  the  supposed  inferior  series  is  associated  with  abundant  material 
of  igneous  origin  which  never  passes  beyond  a  certain  plane,  it  is  al- 
most demonstrative  evidence  of  the  later  age  of  the  newer  series.  A 
notable  instance  of  this  is  found  in  the  Doe  river  section  of  eastern 
Tennessee,  where  the  granitic  rocks  supposed  to  be  older  than  the  as- 
sociated elastics  are  cut  by  very  numerous  schistose  dikes  which  never 
