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.
Rubble Arches. For rough purposes, arches are sometimes built of rubble, as shown in Fig. 58, in which b represents the wall carried by the rubble arch, the ring stones of which, as a, should be narrow and roughly dressed to a wedge shape. Such arches should always be laid in cement mortar, as they depend considerably upon the adhesive power of the mortar for their stability.