BASSLKR.l  CEMENT    MATERIALS    OF    VALLEY    OF    VIRGINIA.  587 
Analysis  of  [Hire  limestone  {Beekmantown)  from  vicinity  of  Staunton,  Va. 
[Charles  Catlett,  analyst.] 
Silica    (SiOa)    1.79 
Alumina    (A1203)   and  iron  oxide   (Fe.Oi) .74 
Lime   (CaO)    50.30 
Magnesia    (MgO)    _ 1.79 
Carbon  dioxide   (0O2) '. 41.  30 
Alkalies,   etc .3.97 
Trenton  limestones. — Under  this  general  name  two  distinct  series 
of  limestone  are  here  recognized — the  older  a  formation  of  coarsely 
crystalline,  highly  fossiliferotis  rock,  and  the  younger  the  well-known 
black  argillaceous  limestones  or  cement  rock.  This  first  series  is  well 
developed  in  the  area  south  of  Staunton,  where  it  varies  from  GO  to 
100  feet  in  thickness,  while  north  of  Staunton  it  is  apparently  missing 
altogether,  for  here  only  the  cement  rocks  occupy  the  interval  be- 
tween the  Beekmantown  limestone  and  Martinsburg  shales.  At  first 
sight  these  two  would  appear  to  be  but  phases  of  one  and  the  same 
formation,  but  this  idea  is  disproved  by  the  development  of  both 
series  in  the  vicinity  of  Lexington,  Va.  Fossils  are  abundant 
throughout  both  series,  the  first  often  being  crowded  with  ramose 
bryozoa  and  masses  of  Solenopora,  while  brachiopods,  ostracoda,  and 
trilobites  of  Trenton  age  predominate  in  the  second. 
Although  samples  of  these  coarsely  crystalline  limestones  run  high 
in  lime,  the  strata  as  a  whole  contain  so  much  chert  that  they  are  of 
little  value  for  mixture  with  the  cement  rock. 
The  }^oungest  member  of  the  Shenandoah  limestone — the  argilla- 
ceous Trenton — usually  resembles  the  corresponding  strata  in  the 
Lehigh  district  more  in  chemical  composition  than  in  its  physical 
aspect.  In  Pennsylvania  the  cement  rock  is  usually  a  dark-gray  or 
black  slaty  limestone,  which,  on  account  of  the  shearing  to  which  it 
has  been  subjected,  breaks  under  the  hammer  into  flat  pieces  with 
smooth  glistening  surfaces.  As  the  rock  loses  its  argillaceous  char- 
acter— i.  e.,  as  the  percentage  of  lime  carbonate  in  it  increases — the 
slaty  appearance  is  lost  and  a  light-gray  crystalline  limestone  is  the 
result.  In  the  Valley  of  Virginia,  however,  metamorphism  seems 
usually  not  to  have  been  so  great  and  the  aspect  of  the  limestones 
varies  according  to  their  composition.  For  example,  the  rocks  of 
the  formation  outcropping  near  Woodstock  are  little  more  than 
compact  hardened  strata  of  calcareous  mud,  while,  on  the  other  hand, 
the  same  horizon  on  the  Avestern  side  of  the  valley  is  occupied  by 
tough,  crystalline,  dark-blue,  or  black  limestones. 
South  of  Staunton,  especially  in  the  vicinity  of  Lexington,  the 
Trenton  strata  have  been  closely  folded  and  compressed,  with  the 
consequent    development   of    considerable    metamorphic    characters. 
