536  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
Analyses  of  pure  limestones  {Cambrian)  from  Annville,  Pa. 
Silica  (Si02)_... 
Alumnia  ( A1203) 
Iron  oxide  (Fe203) 
Lime  carbonate  (CaC03) 
Magnesium  carbonate  (MgC03) 
1. 
2. 
2.14 
3.02 
}     1.46 
1.90 
94.35 
92.05 
2.18 
3.04 
3. 
1.98 
.70 
95.19 
2.03 
Beekmantown  limestone. — The  Cambrian  magnesian  limestones 
grade  upward  imperceptibly  into  another  series  of  strata  having 
essentially  the  same  chemical  composition  but  differing  in  that  exten- 
sive layers  of  chert  are  interbedded  with  the  limestones.  The  areas 
occupied  by  this  division  may  usually  be  recognized  by  their  topo- 
graphic features,  as  the  chert  beds  give  rise  to  conspicuous  hills  or 
ridges.  Chestnut  Ridge,  Sugar  Loaf,  and  Betsey  Bell  are  examples 
of  this  topography  in  the  vicinity  of  Staunton,  but  similar  ridges  and 
knobs  are  encountered  throughout  the  valley.  The  Beekmantown  age 
of  this  series  has  been  determined  from  gasteropod  and  cephalopod 
remains  found  at  various  points  in  the  valley,  but  particularly  in  the 
vicinity  of  Lexington,  Va.  On  account  of  this  gradual  passage  of  the 
Cambrian  into  the  Beekmantown  the  determination  of  the  thickness 
of  the  latter  division  is  difficult.  However,  the  characteristic  fossils 
have  been  found  300  to  400  feet  below  the  top  of  the  cherty  layers, 
so  that  their  thickness  is  not  less  than  the  figures  mentioned.  The 
difficulty  of  separating  these  two  divisions  without  fossil  evidence  is 
in  accordance  with  a  fact  that  has  been  often  noted,  namely,  that 
whenever  both  are  made  up  of  limestone,  sedimentation  has  appar- 
ently continued  through  upper  Cambrian  and  Beekmantown  times 
with  no  interruption. 
Usually  no  pure  limestone  layers  of  any  consequence  have  been 
noticed  in  this  division,  and  this,  as  well  as  the  unfavorable  topog- 
raphy often  accompanying  its  exposures,  causes  the  rocks  of  this  age 
to  be  of  little  value  as  a  source  of  Portland-cement  rock.  Still,  in  a 
few  instances,  lenses  of  comparatively  pure  limestone  have  been  found 
in  this  formation  as  well  as  in  the  underlying  Cambrian.  The  fol- 
lowing analysis  of  a  sample  of  this  rock  from  the  vicinity  of  Staunton 
is  typical : 
