6(30 Geological Society. 



deposit. In Northamptonshire three formations meet these require- 

 ments — the Bhsetic Beds, the Marlstone Bock-bed, and the Oorn- 

 brash. The author applies the Marlstone Bock-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 Bbaatic 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 Bhaetic 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 Bocks and Breccias in Ireland/ 

 By James B. Kilroe, Esq., & Alexander McHenry, Esq., M.B.I.A. 



Many fragmental igneous rocks, although resembling tuffs, can- 

 not be regarded as ejectamenta on account of their character and 

 mode of occurrence in the field. Bocks 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 felsites 

 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 Prof. Lapworth 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 forma- 

 tion of subterranean agglomerates, tuff's, and breccias, which would 

 be forced sometimes between bedding-planes, sometimes into dyke- 

 like fissures.' A series of sections is exhibited to illustrate how 

 tuff-like masses invade black slate of Llandeilo age in the South-east 

 of Ireland, generally adhering to the direction of bedding, but fre- 

 quently cutting across it and detaching numerous pieces from the 

 slate, which are mote abundant near the margins of the intrusion 

 than elsewhere. The masses frequently assume a tuff-like appear- 

 ance. At Arklow Bock tongues of tuff-like rock penetrating black 

 slate of Llandeilo aj. e coutain pieces of Limestone of Bala age, as well 

 as pieces of the shite. The development of vesicular texture in 

 lapilli-like, contained, fragments may be due to the simple release 

 of pressure. 



