This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
106. A very faulty construction sometimes met with is that in which a portion of a ledge of rock projects into the foundation wall, and the foundation is built partly on the rock and partly on the footing course. This is shown in Fig. 39, a being the footing course; b, the rock projection into the foundation wall; c, the thin wall in front of the rock to bring the foundation to the thickness Figured on the plans; and d, the wall of the building carried up to its full height and thickness, and resting partly on the thin wall c, and partly on the ledge of rock b.
In a wall so built, the water will find its way either through the imperceptible seams of the ledge of rock b, or over its top into the body of the masonry, keeping it constantly damp. Besides, there is a serious risk that under the heavy weight of the upper wall, the thin lining built up against the ledge - but in no way bonded to it - would separate from it and fall away, leaving the superincumbent masonry most insecurely supported. There is, besides, the certainty that the foundation wall, built partly on unyielding rock, and partly on softer soil, will settle unequally, and crack, perhaps injuring the masonry above, and, at least, opening an inlet for moisture. The ledge should be cut away so as to leave ample space for the whole thickness of the foundation walls down to the footings, with sufficient space between the wall and the ledge of rock for packing gravel, as shown at a, Fig. 38. This will intercept the water and carry it away from the wall.