The general features of the basaltic district of Ireland have been too frequently described to need more than a few introductory words here. It is situated in the N.E. of Ireland, and forms a plateau with steep escarpments on every side, except in the direction of Lough Neagh. The formation rests upon a very uneven surface, and the thickness of the lowest of the three divisions into which it has been classified is thus very variable. It is described by Hull as “silicated felspathic trachytes, porphyry, pearlstone, pitchstone.” But a good deal of the lava resting upon the Chalk appears outwardly to be amorphous trap. The second division contains all the plant-beds hitherto found in the basaltic formation in Ireland, with the possible exception of those of Lough Neagh. The third is composed of solid sheets of columnar and amorphous basalt. The greatest total thickness observed in Ireland is at Sleamish, a mountain 1437 feet high, all that is visible (at least 1100 or 1200 feet) being composed entirely of basalt. These basalts have been eroded on a colossal scale, for the valleys are scooped out of solid horizontal sheets, as pointed out by Conybeare to this Society so long ago as 1816. They seem to have formed almost the southern limit of a formation which once stretched continuously to Iceland, and to what thickness they were originally erupted can never be known ; but the upper or columnar basalt series, now only 400 or 500 feet thick in Antrim,