The  matter  of  geological  age  is  only  of  very  general  economic  inter-
est. It  is  indeed  true  that  the  process  of  metamorphism — the  change
from  the  amorphous  or  fragmental  condition  to  one  more  or  less  crys-
talline— has  as  a  rule  gone  on  more  extensively  among  the  older
rocks,  than  among  those  later  formed,  but  the  rule  is  by  no  means
universal,  and  moreover  metamorphism  is  not  always  productive  of
such  characteristics  as  make  a  stone  adapted  for  either  building,  monu-
mental or  decorative  work.  While  metamorphism  may  render  a
stone  crystalline,  it  may  also  render  it  granular  and  friable;  while
it  may  develop  color,  it  may  also  develop  schistosity  and  other  blem-
ishes. So  far  as  the  United  States  are  concerned,  one  can  say,  how-
ever, that  few  stones  are  used  to  any  extent  that  are  of  later  date
than  the  Triassic,  and  that  few  if  any  of  our  marbles  are  younger  than
Silurian,  while  nearly  all  our  granites,  as  now  quarried,  belong  at
least  to  Paleozoic  or  Archaean  times.  Stones  of  later  than  Triassic
age,  are,  so  far  as  relates  to  the  Eastern  United  States  so  friable,  or
so  poor  in  color,  as  to  be  of  little  value.
maryland  geological  survey  65

The  Strength  of  Stores.
