1314. Limestones which contain magnesia effervesce with acids less freely than those which contain none; they are also generally known by the presence of numerous minute black dots all over the surface of the stone. From the analysis of Tennant, they appear to contain from 20.3 to 22.5 magnesia; 29.5 to 31.7 lime; 47.2 carbonic acid; 0.8 clay and oxide of iron. Magnesian limestones are usually of a brown or pale yellow colour. They are found in Somersetshire, Leicestershire, Derbyshire, Shropshire, Durham, and Yorkshire; and in many parts of Ireland, particularly near Belfast. In general, when limestones are not magnesian, their purity will be indicated by their loss of weight in burning; the more they lose, the larger is the quantity of calcareous matter they contain. The magnesian limestones contain more carbonic acid than the common limestones; and all of them lose more than half their weight by calcination.