364 CONTEIBUTIONS TO ECONOMIC GEOLOGY, 1902. [nmx.2ia 
and Vermont slates, but are closely related to clay slates. They 
resemble in structure, but not in composition or color, the Welsh 
Penrhyn dark purple ("red") slates/' 
Microscopic examination shows that the slates will cleave less read- 
ily than the Slatington or Vermont " sea-green " slates, and that they 
will be liable to lose some of their blackness on continued exposure, 
and that the amount of discoloration will vary in different beds and 
localities. Although a clay slate even with a small amount of car- 
bonate could hardly prove as durable as a phyllite without any, yet 
it may compare favorably with slates intermediate between phyllites 
and clay slates and containing much more carbonate. 
Slate has been found at the following points in the Martinsburg 
belt: Two miles north 10° west of Middleway, or about 9 miles south of 
Martinsburg, 5 miles northeast, 3£ miles northeast, 2'^ miles southeast, 
2^ and 5| miles south-southeast, 6| miles south-southwest of Martins- 
burg. Numerous other localities will probably be found. The cause 
for the development of slaty cleavage at one point in the shale belt 
and not at another is not yet clear. Thus the railroad and highway 
cuts east of Martinsburg along Tnscarora Creek, which bisects the belt, 
are in shale, but slate occurs north-northeast and south-southwest of 
this along the strike. 
What is needed to develop the industry is to thorough^ open one 
experimental quarry and introduce its product into the market. 
Should that experiment prove successful the growth of the industry 
will be assured. 
aSee Nineteenth Ann. Rept. l T . S. Geol. Survey, Pt. Ill, p. 2H2. 
