Hydraulic cements, such as Portland cement, are made either by grinding and burning natural cement stones - that is, stone containing carbonate of lime or chalk and silicate of alumina or clay - or by grinding together in the we' state clay or mud and chalk, drying, and burning. The materials must be exceedingly fine; that is why Thames and Medway muds are preferred to clay. Paving stones as a rule are composed principally of silica, and are too hard to be ground fine enough. The material might, however, be mixed with Portland cement and moulded Liito artificial stone blocks.