Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/161

 solid and compact, earthy and marly which vary from one to three feet in thickness, and dip north-west at an angle of about 27°. The texture of this limestone is compact, though composed of distinct lamellar concretions: it is traversed by veins of calc spar of a reddish colour, and is soluble in acids without residuum: the specific gravity is 2.84. It is used for building; and the mortar made from it is said to be considerably stronger than that of the newer white limestone. It contains organic remains.

At Desartmartin in the county of Derry, ten miles to the north by east of Cookstown, there is a similar shell limestone of a smoke grey colour, which is also quarried. The strata dip north west at an angle of eight degrees. They contain two sorts of terebratulites, namely, terabratulo giganteo and terabratulo producto, and imbedded nodules of a glassy quartz sometimes of the size of an hazel nut, a circumstance that renders the blasting of this limestone with gunpowder attended with danger. I was informed that at the depth of seven yards, the bed of limestone is exhausted and reposes on a stratum of clay, but I had no opportunity of ascertaining the fact myself.