Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/146

 sides of the channel of the Roe, as is the case at the romantic waterfall called the Dogsleap, near Newton, where a fine natural section of that rock is displayed, and lower down the river in the deer park of Mr. M'Causland; from the district of Tamna Arran near the head of the valley of the Roe, the boundary passes along the Douglas river, a branch of the Mayowla to its confluence: thence it sweeps to the Cast of the Cairns of Slieve Gallion, and near the confines of Moneymore and Lissane, towards the sources of the Ballinderry river, in the north-east of Tyrone.

A little to the north-east of the source of the Roe, and almost surrounded by the secondary and basaltic ridges of Benbradagh and Cragnashoack, we are surprised at meeting with a small insulated district of mica slate; it forms the entire mass of the mountain of Coolcoscrahan, which rises nearly 1300 feet above the level of the sea.

The characters of the mica slate vary much less than might be expected, considering the extent it occupies in the north of Ireland; upwards of two-thirds of it belong to the talcky variety, the remainder to the common, or that which contains the least quantity of mica and the greatest of quartz.

A circumstance rather remarkable is, that, amongst the multiplicity of specimens I have examined, I do not remember one that contained garnets. How far that extensive formation of mica slate may be metalliferous, it is impossible to say in a country hitherto so little explored.

The subordinate rocks which not infrequently occur in the mica slate, shall be noticed separately.