VIRGINIA. 
317 
(vo. Here, however, these limestones are often cut out by the great 
k T er thrust fault of this portion of the valley. 
In this section the best exposures of these rocks are found along 
he eastern edge of Little North Mountain, especially in the vicinity 
f Dry River north of Stokesville. The valley proper, as has been 
pmarked before, is usually occupied by the dolomitic limestones, but 
pcasionally synclines exposing the argillaceous limestones and shales 
re found. The most important of these from an economic standpoint 
beurs just west of Harrisonburg. In the region near Lexington and 
jatural Bridge these limestones have been compressed into close folds 
nd cover much wider areas than they occupy in regions lying farther 
orth. Among these wider areas that in which Lexington is located 
most favorably situated for cement making; the rest, as a glance at 
ne map will show, being too far away from railroad facilities. 
MAKTINSBURG SHALE. 
The highest formation in the valley proper, geologically speaking, is 
great series of gray, light brown or black shales varying in thickness 
■om 1,000 to 1,500 feet. When the Trenton limestones underlie the 
hales the passage from the one formation to the other is often so 
radual that no marked distinction can be observed. Even when the 
hales rest upon older formations than the Trenton argillaceous lime- 
tone, their lower beds are often calcareous and may include thin layers 
f impure limestone. Although the calcareous portion of the shales 
lay burn to cement when mixed with other materials, it is probable 
Ihat the main value of this series will be found in its noncalcareous 
lortions, which may be used for mixture with high lime argillaceous 
ock. The following analyses show the composition of the lower cal- 
areous part and also of the higher, more typical shales: 
Analyses of Martlnsbvrg shales in Virginia, New Jersey, and Pennsylvania. 
!ilica(Si0 2 )-.-. 
Uumina (A1 2 3 ) 
ron oxide ( Fe 2 3 ) 
iime (CaO) 
aleiuni corbonate (CaC0 3 ) .. 
Magnesia (MgO ) 
Magnesium carbonate ( MgC0 3 ) 
Alkalis 
arbon dioxide ( C0 2 ) 
Water (H 2 0) 
68.62 
12.68 
4.20 
2.34 
3.76 
3.73 
4.47 
68.00 
14.40 
5.40 
2.68 
1.51 
.11 
2.30 
2.70 
56.60 
21.00 
5.65 
3.42 
2.30 
.50 
2.20 
3.00 
76.22 
13. 05 
2.67 
93 
23. 08 19. 92 
11.08 10.76 
35. 89 37. 05 
. 94 1. 72 
19.28 
9.86 
36.42 
1.08 
31.70 
1. East Bangor, Pa.. Twentieth Ann. Kept. U. S. Geol. Survey, pt. 6, p. 436. 
2. 1 m. northwest of Colemanville, N. J., Geology New Jersey, 1868, p. 136. 
3. Delaware Water Gap, N. J., Geology New Jersey, 1868, p. 136. 
4. Lafayette, N. J., Rept. New Jersey State Geol. for 1900, p. 74. 
5-7. Calcareous Martinsburg shale, Staunton, Va. (Charles Catlett, analyst. 
