PENNSYLVANIA. 81 
cleavage. To the unaided eye it has a fine texture and a slightly roughish and slightly 
lustrous cleavage surface. It is both graphitic (or carbonaceous) and magnetitic. The 
sawn edge shows pyrite. Under cold dilute hydrochloric acid there is a slight 
effervescence, but more in the ribbon. It is sonorous, and discolors but slightly in 
thirty years exposure. E. H. S. Bailey gives the specific gravity as 2.79. For 
Professor Merriman's recent tests of this slate see page 122. 
Under the microscope the Chapman slate shows a matrix of muscovite (sericite) 
with a brilliant aggregate polarization not obscured by carbonate. A ribbon of 
coarse materials, 0.62 mm. wide, crossing the cleavage at an angle of 14°, possi 
aggregate polarization. There are some large lenses of coarse quartz and carbonate 
in the cleavage. There is considerable carbonate throughout, in plates and rhombs, 
but less than in "Old Bangor" slate. Quartz up to 0.06 mm., pyrite spherules up 
to 0.02 mm. and numbering from 200 to 600 per square millimeter, about 55 scales 
of chlorite, with interleaved muscovite, per square millimeter, lying across the cleav- 
age and measuring generally up to 0.08 by 0.04 mm., graphitic or carbonaceous 
matter in fine particles, abundant rutile needles, rare grains of plagioclase feldspar. 
The ribbon contains more abundant and coarser quartz, carbonate, chlorite, and 
carbonaceous matter than the rest. 
The important constituents, arranged in descending order of abundance, appear to 
be muscovite (sericite), quartz, carbonate, pyrite, chlorite, rutile, carbonaceous 
matter or graphite, magnetite. 
The product of this quarry was formerly used largely for flagging, posts, steps, 
etc., and because of its hardness had to be cut with diamond saws which operated 
horizontally, but owing to the competition of sandstone flagging and the high price 
of diamonds this outlet is closed and the product is now used exclusively for roofing 
slate, for which, however, only selected material is available. 
LEHIGH COUNTY. 
Geological relations. — The slate quarries of Lehigh County all belong in the "soft 
vein" belt. The relation of the "soft vein" slate to the shale of the same formation 
is well exposed near Lehigh Gap, on the border of the county. On the eastern side 
of the gap the outcrop of the Hudson formation that is nearest to the Oneida con- 
glomerate is a black shale that occurs in alternating thick and thin beds. The thick 
ones, being entirely devoid of cleavage, weather into shelly fragments, while the thin 
ones, owing to the intersection of an incipient slaty cleavage with the bedding foli- 
ation, break up into stick-like fragments. Under. the microscope the thin beds prove 
to be an aggregate of muscovite scales, quartz grains, and carbonaceous particles, with 
some pyrite spherules and limonite specks. Here and there the beginning of a slaty 
cleavage foliation is indicated by short stringers of fibrous muscovite lying at right 
angles to the bedding, but otherwise there is no alignment of particles and the rock 
is a typical shale. A hundred feet south of the locality described a southward- 
dipping slaty cleavage predominates, and the northward-dipping bedding isonly shown 
by an occasional ribbon. Therefore within a space of 200 feet the transition from a 
shale to a slate can be observed. In the slate the amount of secondary mica has 
greatly increased and the clastic particles of the shale have been forced into the same 
parallelism of arrangement. 
In weathering, the Lehigh slates undergo the same transformations as those of 
Northampton County. The black slate first passes into a soft yellowish-brown 
ochraceous rock, which later becomes nearly white, and finally passes into a white 
micaceous clay, the "shale clay" of the limonite and fire-clay pits. But these 
changes probably required many centuries. See discussion of chemistry of slate p. 37. 
The Slatington quarries.— The slate quarries of Lehigh County are now confined to 
an area comprising about 3 square miles along Trout Creek and its tributaries. This 
