This region is entirely within Northampton County, and is the most important, in point of production, in the country. The principal quarries are at Bangor, East Bangor and Slatington. The color is a uniform dark blue or blue-black. This slate is used very extensively for blackboards and school slates, as well as for roofing purposes. Average modulus of rupture, 9,810 pounds.
The Lehigh region includes Lehigh County entire and a few quarries in Berk and Carbon Counties and opposite Slatington in Northampton County. The product is similar to that of the Bangor region.
Pen Argyl region embraces quarries at Pen Argyl and Wind Gap in Northampton County.
The Northampton hard-vein region includes the Chapman, Belfast and other quarries, all in Northampton County. This region is distinguished on account of the extreme hardness of the slate as compared with that produced in other regions of the State. The product is considered as the best of the Silurian slates, its extreme hardness being generally considered as an advantage to the slate, rendering it durable and non-absorptive. It is especially suitable for flagging. Average modulus of rupture, about 8,480 pounds.
Peach Bottom Region. - The celebrated "Peach Bottom Slate" is taken from a narrow belt scarcely 6 miles long and a mile wide, extending across the southeastern portion of York County and into Hartford County, in Maryland. The stone is tough, fine and moderately smooth in texture, blue-black in color, and does not fade on exposure, as has been proven by seventy five years' wear on the roofs of buildings. It also ranks very high for strength and durability, and is generally considered equal, if not superior, to any slate in the country. The average modulus of rupture of twelve specimens was 11,260 pounds, the lowest value being 8,320 pounds.
The northern peninsula of Michigan contains an inexhaustible supply of good roofing slate, and extensive quarries have been opened about 15 miles from L'Anse and about 3 miles from Huron Bay. "The stone here is susceptible of being split into large, even slabs of any desired thickness, with a fine, silky, homogenous grain, and combines durability and toughness with smoothness. Its color is an agreeable black and very uniform."*
A good blue-black roofing slate is quarried in Bingham County, Virginia, which bids fair to supplant other slates in that section of the country.
Quarries in Polk County, Georgia, furnish most of the roofing slates for Atlanta and neighboring towns.
Good roofing slate is also known to occur in California, Colorado and Dakota, but the first State mentioned is the only one in which quarries have yet been opened.
* " Stones for Building and Decoration," p. 302.