The durability of sixty-eight shale samples from the Appalachian region was measured using the Jar Slake (Soak) Test. Of the sixty-eight samples tested, fourteen degraded into a pile of flakes or mud, four developed small fractures, and fifty experienced no degradation at all. X-ray diffraction analysis identified kaolinite as the predominant clay mineral present in the shales. Shales composed primarily of kaolinite slake as a result of pore air compression. Pore air compression is favored by small pore radii. Thin section analysis found the fourteen shales that slaked had a system of macropores with an average diameter equal to 0.06 mm, the four samples that developed small fractures had macropores with an average diameter equal to 0.07 mm, and the fifty samples that experienced no degradation at all had macropores with an average diameter equal to 0.092 mm. This indicates that the size of the macropores has a definite influence on the slaking of the shales.