164 
PROCEEDINGS OF SOCIETIES. 
Dr. E. Percival Wright opposed the idea that Posidonia was an 
Entomostracan shell, or that it was at all allied to Aplysia; on the 
contrary, he believed it to have been a molluscous animal, whose affinities 
approached very near to those of Avicula; and he thought that the spe¬ 
cimen exhibited by Sir E. Griffith completely settled the question. 
Professor Haughton believed that the strata between Eush and the 
Skerries could not be more than 200 feet thick, and that the mud strata 
in which Posidonia and the Goniatites occur were identical. These mud 
bands occurred between strata of crinoid limestone, in which Eenestellae 
are found. He thought that these mud bands were of fiuviatile origin, 
their dark colour being due to the presence of plants, the conglomerate 
accompanying them indicating a sea shore. The various species of Po¬ 
sidonia might be, perhaps, referred to one, when the distortion due to 
cleavage was taken into account. 
Mr. J. Eeete Jukes fully concurred with Sir Eichard Griffith as to 
the great rarity of finding both valves of Molluscan shells, even in a 
recent state; he had not been able accurately to refer the Conglomerate 
of Eush to either the upper or lower limestone series. 
Lord Talbot de Malahide then took the Chair. 
The President read a paper— 
on reversed faults occurring in anticlinal folds with oblique axes, 
ILLUSTRATED BY A CASE AT LOUGH SHINNY, COUNTY OF DUBLIN. 
Having recently had occasion to study somewhat carefully the condi¬ 
tions under which reversed faults occur, and having arrived at some 
results, which, so far as I know, have not been published hitherto, I 
thought it might be useful to lay them before the Society, in the hope 
that the circumstances under which reversed faults occur may be noted 
occasionally, and thus more facts collected, on which to construct a 
complete theory of the forces which have given rise to them. 
It is well known to engineers that if a bank of earth be retained by 
a vertical revetment wall, there is a certain plane, which may be called 
the natural plane of slipping , along with there is the greatest tendency 
of the bank to slide. If u denote the underlay (or angle with the ver¬ 
tical) of this plane, and 0 the angle of friction between the masses of 
which the bank is composed; then— 
0 
« = 45°-|. (1) 
If, from any cause, the bank of earth should tend to slip down some 
other plane, not the natural plane of slipping , then the horizontal force 
necessary to keep it from so slipping is, 
P- ±/ix 2 tan u cot (u+ <(>); (2) 
where g denotes the weight of a cube foot of the bank, x the height of 
the revetment wall, and P the force per linear foot of breadth requisite 
to keep the bank from slipping. 
