SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 75 
PENNSYLVANIA. 
By T. Nelson Dale. 
GENERAL FEATURES. 
The slates of Pennsylvania, aside from those of Lancaster County and the south- 
astern part of York County, which are described under the heading " Peach Bottom " 
ate (pp. 85-88), occur in Northampton and Lehigh counties in a strip from 2 to 4 
liles wide on the southern side of the Blue Mountain, extending from Delaware 
i^ater Gap in a west-southwest direction to a point 4 miles west of Lehigh Gap, a 
istance of about 32 miles. The chief centers of the slate industry here are at Bangor 
rid Slatington. The geographical relations of these places and of the other groups 
I slate quarries in these counties are shown on the maps of the Second Pennsylvania 
leological Survey of Merrill's Stones for Building and Decoration (third edition) 
rid of Merriman's article in Stone for July, 1898. 
The general geological relations of this slate are these: On the southeast, forming 
ri east-northeast to west-southwest belt between Easton and Reading and beyond, 
re the pre-Cambrian gneisses, etc., of South Mountain, flanked and dotted over with 
;rips of Lower Cambrian quartzite and sandstone. Northwest of this and parallel 
) it is a great Cambro-Ordovician dolomite and limestone plain from 3 to 6 miles 
r ide, under which the quartzite dips. Still farther northwest is a slightly hilly belt 
f Ordovician shales, grits, and roofing slates of Hudson age, from 6 to 8 miles wide, 
.t the southeast these shales and slates overlie the limestone, and at the northwest 
ley dip under the Silurian conglomerate and sandstone of the Blue Mountain. The 
oundary between the shale and slate formation and the limestone is roughly parallel 
) the general course of the Oneida and Medina boundary, but passes a little north of 
Nazareth. 
The shale and slate formation measures from a minimum exposure of 1,600 feet to 
n estimated maximum of 6,000 feet, of which the commercial slate comprises but a 
;w hundred feet. In structure this formation consists of a succession of minor close 
)lds, generally overturned to the northern horizon, so that their axial planes have 
southerly dip but usually so that the synclines have a steeply inclined southern and 
gently inclined northern limb. Sometimes the fold is extremely close and the 
verturn so complete that its axial plane has a very low southeasterly dip. The 
leavage dips to the southern horizon at various angles, sometimes at one of 5°, point- 
lg, like the curvature of cleavage and jointing, to a secondary movement. These 
)lds vary greatly in width and their axes also pitch alternately east-southeast and 
rest-northwest at angles ranging from 5° to 10°. They also bend laterally from 
orth to south. These folds have been more or less truncated by surface erosion, and 
l places the cleavage foliation has heen crushed and bent over to the south by the 
"iction of the southward-moving ice sheet. 
" Ribbons," or small beds of grit, measuring from a fraction of an inch to two feet 
l thickness, characterize the slate belt throughout. This grit consists mostly of 
lore or less angular grains of quartz and feldspar, rarely of shale and quartzite, 
Dgether with scales of muscovite and lenses of chlorite, spherules of pyrite, carbon- 
ceous matter, all in a cement of calcite and sericite with rhombs of carbonate more 
r less altered to limonite. The ribbons, as explained on page 6, represent coarse 
Bdiments, brought in, probably, by local marine currents. 
Commercial slate is obtained along two belts. The upper and northerly one, known 
s the "soft vein," which is separated from the overlying Silurian conglomerate by 
n uncertain thickness of shale and slate, consists of beds of relatively soft slate of 
Limcient thickness between the ribbons to furnish large slabs suitable for mill stock 
r roofing purposes. The lower and southerly belt, the "hard vein," near the base 
