Page 35 - History of Rockingham Co

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Late investigations have revealed

that igneous rocks also occur in the

county within the Valley province,

in limited quantity, in the form of

dikes. These rocks outcrop in the

vicinity of Cross Keys and at intervals

entirely across the county; the most

westerly outcrop reported being in the

Avicinity of Brock's Gap. character-

istic exposure of these rocks is found

near Harrisonburg, where the road

leading from Harrisonburg to Keezle-

town crosses the Chesapeake - Wes-

tern Railway tracks. The rock is

/farrmttmy of dark color and medium to fine

grained. It weathers to rounded boul-

ders, which are broken only with great

difficulty. It is a typical diabase and

is composed of the minerals magnetite,

augite, olivine, and plagioclase feld-

spar as essential ingredients. The rock

is comparatively fresh, and is prob-

ably the youngest of the rock forma-

tions in the county, having been forced

while in a molten condition into the

fissures of the limestone and shale

formations of the mountains. While

the exact age of the rock cannot be de-

termined, on lithological grounds it

mayaoutfntHt. be assigned to the Triassic. ^ The

rock, owing to its high lime and iron

content, makes road material of the

first quality, since it has the necessary

ingredients to bind and is far superior

to limestone in lasting qualities.

The sedimentary rocks of the county

present every phase of sedimentary

1- Thos. L. Watson and Justus H. Cline,
"Dikes of the ShenandoahValley, " in prepa-

ration.