668  PKE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
in  width,  and  strikes  northeast-southwest.  The  granite,  gneiss,  and 
schist  are  regarded  as  pre- Cambrian.  The  slates  resemble  the  roofing 
slates  on  James  Kiver,  which  carry  Lower  Silurian  fossils. 
Keith,36  in  1894,  gives  the  geology  of  the  Catoctin  belt.  The  pre- 
Cambrian  rocks  constituting  the  Blue  Ridge  core  are  all  of  igneous 
origin  and  are  assigned  to  the  Algonkian.  They  include  quartz 
porphyry  and  andesite,  Catoctin  schist,  and  granite.  A  detailed 
lithological  description  is  given  of  each  of  these  rocks  and  of  their 
alterations.  The  Catoctin  schist  and  the  granite-  are  separated  by 
areas  in  which  the  two  are  intimately  intermingled.  The  Catoctin 
schist  is  an  altered  diabase,  and  the  diabase  is  believed  to  be  separable 
into  two  flows  with  a  time  gap  between  them.  An  evidence  of  this 
is  a  discordance  of  structure.  The  order  of  the  events  was  probably 
as  follows :( 1 )  Diabase  extrusion,  (2)  granite  intrusion,  (3)  erosion 
interval,  (4)  quartz  porphyry  and  andesite  flows,  (5)  erosion  inter- 
val (?),  (6)  diabase  flow,  and  (7)  erosion  interval.  The  different 
lavas  have  been  folded  and  faulted,  and  secondary  structures  have 
developed  within  them.  Metamorphism  was  most  extensive  in  the 
diabase,  which  has  become  a  well-developed  schist.  The  quartz  por- 
phyry is  the  least  altered. 
Merrill,37  in  1895,  describes  the  disintegration  of  the  granite  rocks 
in  the  District  of  Columbia,  and  finds  from  chemical  analysis,  calcu- 
lated on  a  water-free  basis,  that  they  are  very  similar  to  those  of  the 
original  rocks,  and  therefore  that  the  rocks  are  as  much  disintegrated 
as  decomposed.    The  chief  chemical  change  is  hydration. 
Keyes,38  in  1895,  gives  a  detailed  petrographical  description  of  the 
Maryland  granites.  For  reasons  the  same  as  given  by  Williams  they 
are  regarded  as  eruptive,  and  many  of  the  gneisses  are  shown  to  be 
dynamically  metamorphosed  granites. 
Williams,39  in  1895,  considers  the  general  relations  of  the  granitic 
rocks  in  the  middle  Atlantic  Piedmont  Plateau  and  maps  the  same. 
The  criteria  by  which  ancient  plutonic  rocks  in  highly  metamor- 
phosed terranes  may  be  recognized  comprise  radiating  dikes,  inclu- 
sions of  fragments,  contact  zones,  chemical  composition,  and  petro- 
graphical structure.  On  these  criteria  it  is  concluded  that  most  of 
the  granitic  rocks  of  Maryland  are  igneous,  although  many  of  them 
are  changed  to  granite  gneiss,  and  of  certain  of  these  gneisses  it  can 
not  be  asserted  whether  they  are  of  aqueous  or  of  igneous  origin. 
South  of  Laurel,  in  the  large  area  from  Triaclelphia  southward  to 
Brookville,  at  Murdoch  Mill  west  of  Washington,  south  of  Falls 
Church  in  Fairfax  County,  Va.,  and  at  Cabin  John  Bridge  on  Poto- 
mac River  there  are  gradations  between  granitic  rocks  and  diorites  or 
gabbros.  In  the  Maryland  rocks  pegmatites  are  abundant.  Some  of 
these  are,  as  indicated  by  their  association  with  quartz  veins  and  by 
