e*ale.] SLATE INDUSTRY OF PENNSYLVANIA-WEST VIRGINIA. 363 
be almost a slate. It is possible that a dark red clay slate, suitable 
,for roofing, oecurs below the top roek in that vicinity. 
MARTINSBURG, W. VA. 
This recently prospected slate district lies in Berkeley County, W. 
Va., within the geologic belt designated Martinsburg shale in the 
Harpers Ferry folio. This belt lies about 13 miles west of the Blue 
Ridge and mostly on the western side of Opequon Creek, a tributary 
of the Potomac. It measures at least 14 miles in length, from north- 
northeast to south-southwest, and from 2 to -1 in width. Martinsburg 
lies just beyond its western edge. 
This shale and slate formation, estimated to be from 700 to 1,000 feet 
in thickness, is of Lower Silurian age, and overlies the Siluro-Cambrian 
Shenandoah limestone in a series of folds represented in the folio 
as overturned to the west. The rock is generalty a dark grayish 
shale, weathering into a yellowish or white clay, known locally as 
"soapstone." The general character of this rock and its appearance 
when weathered would hardly be regarded as good indications of the 
presence of roofing slate. But at several points, usually near the 
Opequon or its tributary "runs" or creeks, where the mass has been 
denuded of its weathered portions, it has a well-marked easterly dip- 
ping' (in one case, westerly) slaty cleavage crossing the bedding at 
various angles; and pieces, when struck with the hammer, give the 
typical ring of a slate. A superficial examination of this slate shows 
that its cleavage is far from being as fine as that of the Slatington 
quarries and that the cleavage surface, although quite as black, yet 
lacks the smoothness and luster of the Slatington product, On the 
other hand, it effervesces far less readily or not at all with cold dilute 
hydrochloric acid. Its relative commercial value will be soon deter- 
mined scientifically by the usual physical and chemical tests and by 
comparing the results of these tests with those obtained from tests of 
"Peach Bottom" slates. Meanwhile preliminary microscopic exam- 
inations of specimens taken from several localities have been made 
with the following results. 
All the transverse sections show a rather coarse and poorly defined 
cleavage and a very faint polarization of the matrix, in some cases 
none at all, indicating incomplete sericitization. All the sections 
show carbonate, some in very small amount, others in large. Car- 
bonaceous matter is present in all the sections, but is less conspicu- 
ous than in the Slatington slates. The following minerals are also 
present: pyrite in spherules, angular quartz grains, plagioclase grains 
very rarely, rather large muscovite scales, chlorite usually interleaved 
with muscovite, and the usual slate needles (Ti0 2 ), but these are not 
as abundant as in other slates. 
The above suffices to show that these slates are neither true phyl- 
lites nor midway between phyllites and clay slates, like the Slatington 
