SUMMARY   OF   GENERAL   LITERATURE.  65 
recognized  as  classes,  as  they  may  occur  in  all  series  from  the  earliest 
to  the  latest  sediments.  That  gneiss,  mica  slate,  hornblende,  talcose 
slate,  etc.,  are  metamorphic  altered  sediments  there  is  no  evidence. 
Azoic  is  objectionable  because  it  presupposes  that  our  observations 
have  made  certain  that  which  must  ever  remain  doubtful. 
The  Pyrocrystalline  rocks  are  due  to  the  consolidation  of  the  earth's 
crust.  These  rocks  increase  in  thickness  by  additions  below.  On 
contract  ion  fissures  are  formed,  through  which  flow  fluid  materials 
which  are  Hydroplastic  and  later.  The  age  of  rocks  may  be  deduced 
from  the  perfection  of  their  crystalline  state.  The  preeminently 
crystalline  granites  are  the  product  arising  from  the  first  cooling  of 
the  first  crust.  Granite  may  or  may  not  be  connected  with  the  oldesi 
masses  of  the  globe.  The  granites  of  the  United  States  are  of  two 
classes;  those  of  one  class  are  more  ancient  than  the  Taconic  rocks; 
the  others,  which  are  later  eruptions,  are  certainly  as  new  as  the 
Carboniferous.  In  New  England  the  ancient  granites  are  Avidespread, 
hut  there  are  also  large  areas  of  granite  which  has  been  erupted  from 
fissures  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  system,  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 
Potsdam  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 ;  but  a  comparison  of  the  lower  members  of  the  Taconic  and 
the  Silurian  shows  a  decided  difference  in  mineral  constitution.  The 
first  partakes  of  the  primary  character  of  the  granite,  gneiss,  mica, 
and  talcose  slates  of  the  Pyrocrystalline  rocks,  from  the  last  two  of 
which  it  is  often  difficult  to  distinguish  them,  while  the  materials  of 
the  Silurian  are  derived  from  the  Taconic  rocks.  The  Taconic  sys- 
tem, from  the  base  upward,  comprises  (1)  conglomerates  and  brec- 
cias, (2)  limestones,  (3)  slate  of  enormous  thickness,  (4)  dark-colored 
Taconic  slates,  and  (5)  sparry  limestones.  The  absence  of  fossils 
in  the  Taconic  rocks  is  thought  to  be  due  to  the  probable  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  Ta- 
55721— Bull.  360—4)9 5 
