544 
CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
Lexington. — In  point  of  having  an  abundance  of  argillaceous  lime- 
stones close  at  hand,  Lexington  is  most  favorably  placed,  as  it  is 
situated  in  the  midst  of  a  broad  area  of  these  rocks.  In  this  part  of 
the  valley  the  Trenton  limestones  have  been  closely  folded  and  over- 
turned to  the  west,  so  that  the  exposures  seem  to  show  an  extraordi- 
nary thickness.  Occasionally  the  core  of  an  anticline  or  syncline  may 
be  noted,  and  whenever  it  is  possible  to  make  accurate  measurements 
the  thickness  of  the  formation  is  found  not  to  exceed  350  feet.  The 
composition  of  this  rock  is  such  that  theoretically  it  ought  to  make 
a  high-grade  Portland  cement,  but,  as  noted  by  Catlett,  it  is  a  ques- 
tion whether  the  relatively  high  ratio  of  silica  to  iron  and  alumina, 
tending  to  increase  the  refractory  character  of  the  clinker,  is  offset 
by  the  finely  divided  condition  and  intimate  mixing  of  the  natural 
material.  The  following  analyses  show  the  variation  in  the  compo- 
sition of  the  rocks : 
Analysis  of  Trenton  limestones  from  Lexington,  Va. 
[Charles  Catlett,  analyst.] 
Silica  (Si02) 
Oxides  (A1203,  Fe203) 
Lime  (CaO) __. 
Magnesia  (MgO) 
Carbon  dioxide  (C02) 
1. 
2. 
B. 
4- 
5. 
0.73 
9.31 
11.86 
12.92 
17.42 
.79 
3.47 
1.76 
3.88 
4.70 
53.71 
46.30 
46.64 
45.14 
42.44 
.83 
.86 
.74 
1.37 
1.68 
38.32 
37.  20 
35.62 
22.60 
7.06 
36.72 
1.69 
32.52 
Analysis  of  Trenton  limestone,  5  miles  east  of  Woodstock,  Va. 
Silica  (Si02)  16.34 
Alumina   (A1203)   and  iron  oxide   (Fe203) 7.49 
Lime  carbonate   (CaC03)    74.14 
Magnesium  carbonate  (MgC03) 1.00 
Water  (H20)   2.00 
