van  HisE.]  DISCUSSIONS    OF    PRINCIPLES.  441 
crust.  These  rocks  increase  in  thickness  by  additions  below.  On  the 
contractions  of  these  rocks  fissures  are  formed,  through  which  flow 
fluid  material  which  are  hydroplastic  and  later.  The  age  of  rocks  may 
be  deduced  from  the  perfection  of  their  crystalline  state.  The  pre 
eminently  crystalline  granites  are  the  product  arising  from  the  first 
cooling  of  the  first  crust.  Granite  may  or  may  not  be  connected  with 
the  oldest  masses  of  the  globe.  The  granites  of  the  United  States  are 
of  two  classes;  one  more  ancient  than  the  Taconic  rocks,  and  others, 
which  are  later  eruptions,  certainly  as  new  as  the  Carboniferous.  In 
New  England  the  ancient  granites  are  widespread,  but  there  are  also 
found  granitic  areas  which  have  been  erupted  from  fissures  and  which 
have  overflowed  wide  areas  and  whose  structure  is  more  or  less  sheeted. 
It  is  impossible  to  draw  lines  of  distinction  between  the  two  kinds  of 
granite,  except  when  the  earlier  granite  is  traversed  by  the  later.  The 
lamination  of  the  laminated  Pyrocrystalline  rocks  is  probably  due  to 
crystallization.  Gneiss,  mica- slate,  hornblende,  and  talcose  slate  are 
so  blended  that  it  is  difficult  or  impossible  to  define  their  boundaries, 
and  they  are  all  regarded  as  contemporaneous  formations. 
The  oldest  Hydroplastic  rocks  constitute  the  Taconic  sy stein,  which 
has  a  clear  and  well  defined  base,  rarely  obscured  by  passages  into  the 
primary  schists  or  the  Pyroplastic  syenites  or  granites.  This  system  is 
limited  above  by  the  Silurian  system,  at  the  base  of  which  is  the  Pots- 
dam sandstone.  The  thickness  of  this  system  is  from  25,000  to  30,000 
feet.  Sediments  of  all  systems  must  necessarily  consist  of  the  same 
materials.  Sandstones,  limestones,  slates,  conglomerates,  and  breccias 
must  make  up  the  matter  which  composes  them;  but  a  comparison  of 
the  lower  members  of  the  Taconic  and  the  Silurian  show  a  decided 
difference  in  mineral  constitution.  The  first  partakes  of  the  primary 
character  of  the  granite,  gneiss,  mica,  and  talcose  slates  of  the  Pyro- 
crystalline rocks,  from  the  last  two  of  which  it  is  often  difficult  to  dis- 
tinguish them,  while  the  materials  of  the  Silurian  are  derived  from  the 
Taconic  rocks.  The  Taconic  system,  from  the  base  upward,  comprises 
(1)  conglomerates  and  breccias;  (2)  limestones;  (3)  slate  of  enormous 
thickness;  (4)  dark -colored  Taconic  slates;  (5)  sparry  limestones.  The 
absence  of  fossils  in  the  Taconic  rocks  is  thought  to  be  due  to  the  prob- 
able absence  of  animals  and  plants  at  the  time  the  Berkshire  limestone 
and  earlier  Taconic  rocks  were  deposited.  In  the  Upper  Taconic  rocks 
are  found  remains  of  both  plants  and  animals.  The  rocks  of  the 
Taconic  system  form  a  belt  on  both  sides  of  the  Blue  ridge.  On  the  west 
it  is  continuous  from  Canada  East  to  Georgia.  On  the  east  it  is  wider 
in  certain  pi  aces  than  on  the  west  side,  but  its  continuity  i  s  broken.  Th  e 
Taconic  system  rests  then  on  the  following  points:  The  formations  of 
the  series  are  physically  unlike  the  Lower  Silurian;  it  supports  the 
Lower  Silurian  unconformably  at  numerous  places;  it  is  a  system  in 
which  life  existed,  and  the  remains  of  organisms  are  left  which  are 
unlike  those  of  the  Lower  Silurian;  it  carries  back  many  stages  farther 
