Reports and Proceedings — Geological Society of London. 381 



situated as near as possible, in vertical sequence, to the reference 

 deposit. In Northamptonshire three formations meet these require- 

 ments — the Rha^tic beds, the Marlstone Rock-bed, and the Corn- 

 brash. The author applies the Marlstone rock-bed as a datum to 

 the study of the five chief deep explorations in Northamptonshire, 

 with the following results : — While the old land-surface (below the 

 Trias) now varies in height by more than 250 feet, the variation in 

 thickness of the rocks between it and the Middle Lias only reaches 

 56^ feet ; and although the old land-surface is actually lowest 

 where the Rhfetic rocks have not been detected, when compared 

 with the position of the Marlstone it is found to be the highest. 

 The further application of the same method enables the author to 

 recognize Rhsetic rocks at Northampton, to correct the record of 

 the Kingsthorpe shaft, and to explain the presence of Triassic saline 

 water in the Marlstone. A revised section of the Kingsthorpe shaft 

 is given. Another point proved is that a general levelling-up 

 process was going on just before the beginning of the Lower Liassic 

 Period, and another at the close of the Middle Liassic Period. 



2. "On Intrusive, Tuff-like, Igneous Rocks and Breccias in 

 Ireland." By James R. Kilroe, Esq., and Alexander McHenry, Esq., 

 M.R.I.A. (Communicated by R. S. Herries, Esq., M.A., Sec. G.S., 

 with the permission of the Director of H.M. Geological Survey.) 



Many fragmental igneous rocks, although resembling tuffs, cannot 

 be regarded as ejectamenta on account of their character and mode 

 of occurrence in the field. Rocks of this type occur to the east 

 of Lough Eake in Donegal, in the district of Forkhill in Armagh, 

 at Blackball Head in Cork, in Waterford, near Arklow, in Wexford, 

 and elsewhere. Sometimes they consist of partly fused and broken- 

 up felspathic mica-schist merging into felsite-dykes, at other times 

 of brecciated slate, granite, and felsite embedded in a scanty andesitic 

 matrix. At Blackball Head the rocks cross the bedding of the 

 associated sedimentary rocks of the region. The authors agree with 

 Professor Lap worth in considering it possible that " igneous matter 

 making its way between the moving masses may consolidate as 

 sills when the pressure is great, , , . As movement progressed 

 intermittently, we should have the formation of subterranean 

 agglomerates, tuffs, and breccias, which would be forced sometimes 

 between bedding-planes, sometimes into dyke-like fissures," A 

 series of sections is exhibited to illustrate how tuflf-like masses 

 invade black slate of Llandeilo age in the south-east of Ireland, 

 generally adhering to the direction of bedding, but frequently 

 cutting across it and detaching numerous pieces fi'om the slate, 

 which are more abundant near the margins of the intrusion than 

 elsewhere. The masses frequently assume a tuff-like appearance. 

 At Arklow Rock tongues of tuff-like rock penetrating black slate 

 of Llandeilo age contain pieces of limestone of Bala age, as well as 

 pieces of the slate. The development of vesicular texture in lapilli- 

 like, contained, fragments may be due to the simple release of 

 pressure. 



