Anniversury Address to the Royal Geological Society. 493 
the “Bunter Sandstone,” while in the co. Tyrone the rocks in 
immediate contact have the character of the “ Keuper Marl.” 
Above the Triassic are the “ Rhietic,” or the passage beds from 
them into the Jurassic. In those places where the upper beds 
of the Rheetic have not been removed by denudation, they form 
a complete sequence between the Triassicand the Jurassic, although 
their thickness is in some places less than 20 fect, increasing on 
the north side of Belfast Lough to only something over 100 feet. 
Evidently they thin away south-westward and westward, as do 
also the underlying Triassic rocks. 
The Jurassic rocks are only represented by a portion of the 
Lias, and that but sparingly, as the greater portion of these 
rocks seems to have been removed by denudation prior to the 
deposition of the Cretaceous strata. Enough, however, of them 
are preserved to show their relations to the underlying Rheetic, 
and to prove the thinning away of the latter towards the west- 
ward and south-westward. 
The rocks belonging to the Ivish Permian, Triassic, and Jurassic 
were evidently deposited respectively in waters of very unequal 
depths. In the neighbourhood of Belfast Lough the sequence 
consists of more members than in the counties of Tyrone and 
Londonderry, besides being of a much greater thickness. The 
thicknesses, however, suddenly vary in the latter area, as near 
Stewartstown, co. Tyrone, the Trias cannot be more than thirty 
feet thick, while a little to the north at Croagh it is over 280 
feet, but a little farther northward, in Londonderry, it is less 
than 50 feet. 
Belfast Lough. Tyrone and Londonderry. 
5. Lias. Lias. 
4, Rhetic, maximum 100 feet Rhetic, about 25 feet 
3. Keuper, * 1000 ,, } Keuper. Very variable, maximum 
2. Bunter, - 1000 ,, 5 about 300 ,, 
1. Permian. Permian. 
In both these districts the sequences ought to represent the 
same period of time, that is, from the Permian to the Lias, unless 
indeed, which is not impossible, the Permians are only “shore 
beds” of the Triassic. In this case the Tyrone Permians may 
have accumulated on a higher horizon than those in the vicinity 
of Belfast. That in both areas there are continuous regular 
