512  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  (bull. 86. 
been  thought  to  be  characteristic  of  the  period.  At  other  times  the 
abundance  of  volcanic?  material  has  been  the  reason  for  the  reference. 
Again,  quartzites  from  the  Appalachians  to  the  Black  hills  have  been 
correlated  on  no  other  ground  than  lithological  likeness,  as  though 
thick  sandstone  formations  which  subsequently  have  been  cemented  to 
<  i  uartzites  have  been  produced  but  once  in  the  history  of  the  world. 
To  some  geologists  the  degree  of  crystallization  has  been  the  control- 
ling fact.  In  other  cases  the  occurrence  of  some  mineral  or  associa- 
tion of  minerals  has  been  the  gronnd  upon  Avhich  the  reference  of  the 
containing  formation  is  made  to  some  specific  period.  This  has  gone 
so  far  at  times  as  to  lead  to  the  conclusion  that  a  single  rare  mineral, 
such  as  chondrodite,  is  proof  of  the  pre- Cambrian  age  of  the  contain- 
ing formation. 
As  a  natural  result  of  work  of  this  kind,  rocks  of  a  certain  lithological 
character  or  degree  of  crystallization,  or  containing  certain  constituents, 
have  been  called  Arvonian,  Huronian,  Norian,  Laurentian,  as  the  case 
may  be,  which  have  afterwards  proved  to  be  high  in  the  Paleozoic. 
Other  series,  which  are  now  known  to  be  pre- Cambrian,  have  been 
called  Triassic,  because  of  a  prevailing  red  hue. 
Any  of  the  above  characteristics  may  be  a  valuable  guide  in  a  given 
district,  or  with  qualifying  and  guiding  facts  of  a  different  character 
in  a  region  5  but  it  is  their  use  in  an  indiscriminate  manner  on  the 
assumption  that  rocks  of  a  given  time  are  every  where  alike  that  is  pro- 
tested against.  When  it  is  everywhere  recognized  that,  considering 
the  continent  as  a  whole,  age  is  no  guide  to  the  chemical  or  mineral 
composition,  texture,  color,  degree  of  crystallization,  or  any  other  prop- 
erty of  a  formation,  or  vice  versa,  we  shall  be  on  the  way  to  use  the 
properties  of  rocks  in  districts  and  regions  as  guides  to  age.  For  the 
most  part  this  principle  has  been  recognized,  if  not  practically,  at  least 
theoretically;  but  at  one  point  this  is  less  true  than  with  the  others. 
Degree  of  crystallization,  because  often  so  useful  in  a  district,  has  been 
used  by  many  as  a  general  guide  in  correlation,  although  the  elder 
Hitchcock,  Rogers,  Adams,  and  others  gave  early  warning  against 
such  practices.  It  can  not  be  too  strongly  insisted  upon  that  contact 
action  of  great  masses  of  eruptive  rocks  and  dynamic  action  accom- 
panying this,  or  dynamic  action  without  accompanying  volcanic  activ- 
ity, are  prime  and  perhaps  the  chief  causes  in  the  majority  of  cases  of 
the  production  of  crystallization,  not  age  and  depth  of  burying,  although 
these  may  be  contributory  causes  and  at  times  the  predominant  ones. 
A  stratum  which  is  strongly  conglomeratic  at  the  axis  of  a  fold,  within 
a  short  distance  upon  the  legs  may  have  become  so  completely  crystal- 
line as  to  obliterate  every  trace  of  original  fragmental  character, 
because  of  the  movement  of  the  particles  over  each  other,  as  for  instance, 
in  the  Cambrian  at  Hoosac  mountain.  In  strong  contrast  to  this  may 
be  mentioned  the  occurrence  of  quartzites  in  the  lower  part  of  the 
Upper  Huronian  of  lake  Superior,  which  show  no  evidence  whatever 
