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.
53. In placing foundation footings on rock, it is sometimes found that some portions of the footings will rest on the rock, and others, owing to the diversified character of the surface, will rest on clay, sand, or gravel. This settlement of the foundation walls - and as a necessary sequence, that of the whole building - will then be uneven, as the walls resting on the rock will not settle, while those resting on the sand, gravel, or clay will, by compressing the material on which they are carried.
Fig. 16 shows the method used to obtain equal settlement. At (a) is shown the rock and gravel before leveling or excavating; a indicating the clay or sand, and b, the rock. It is customary to remove the rock to a certain level, as shown in (b). The softer soil b is then removed and leveled off, as at b b, and a bed of concrete about 3 feet thick, as shown at c, is then put down, the concrete being brought to the level of the rock, and on this the brick or stone foundation wall d is built.
54. It is not considered necessary, on solid rock, to have the footing bed cut level over its entire surface, nor even cut into a series of horizontal surfaces resembling steps, as is frequently done in softer soils, which method costs a great deal of time and money; but it is necessary that the surface of the rock shall be so roughened that the possibility of the footing slipping on its foundation will be prevented. After this is done, concrete may be put in to bring the foundation to its proper level. When the structure is three or four stories in height, stone or brick may be used in place of concrete, but a concrete base is usually considered preferable.