This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The preparation of a rock bed on which a foundation is to be placed, is a simple matter compared with that required for some soils on which foundations are placed. The bed-rock is prepared by cutting away the loose and decayed portions of the rock and making the plane on which the foundation is placed perpendicular to the direction of the pressure. If the rock bed is an inclined plane, a series of steps can be made for the support of the foundation. Any fissures in the rock should be filled with concrete.
Whenever it is necessary to start the foundation of a structure at different levels, great care is required to prevent a break in the joints at the stepping places. The precautions to be taken are that the mortar-joints must be kept as thin as possible; the lower part of the foundations should be laid in cement mortar; and the work should proceed slowly. By following these precautions, the settlement in the lower part will be reduced to a minimum. These precautions apply to foundations of all classes.