10 Scientific Proceedings. Royal Dublin Society, 



At the northern end the beds disappear under the sand of the Malahide 

 inlet, The next exposure in this direction occurs at a distance of one and 

 a quarter mile, at Portraine, and consists of Bala rocks. 



To the south the rocks are covered by the Velvet Strand of Portmarnock, 

 the next considerable exposure being the Lower Carboniferous of Howth, 

 three miles away. There is, however, in certain states of the sand, a reef of 

 nodular limestone uncovered 250 yards S. of the continuous exposure. 



At Carrickhill the actual fault is concealed by sand. The strike of the 

 fault is about N. 30° W. On the N.E. side the beds have a dip to the IST.W., 

 except in close proximity to the fault, where they bend round rather sharply 

 and dip towards the fault, making it almost certain that this is the up-throw 

 side. 



S.W. of the fault the beds are greatly disturbed, the shaly beds being 

 twisted into zig-zags, the limestones squeezed into lenticles, or fractured, 

 and full of calcite veins. A series of parallel displacements of slight throw, 

 having nearly the same strike as the fault, occur at intervals down to the 

 S. end of the section. These all have the down-throw to the S.W.,i which 

 supports the view that this is the direction of throw in the main fault. The 

 amount of stratigraphical throw is estimated at over 780 feet (see p. 17). 



The C.G. fault strikes E. and W., and is seen in the cliff to hade to the S. 

 The beds immediately to the N. of it are dolomitized, and form a gentle 

 anticline, thus dipping towards the fault. Immediately S. of the fault the 

 beds are shattered, and full of calcite veins. There is also some dolomitiza- 

 tion for a few feet. In places the beds on this side can be seen to be sharply 

 upturned in contact with the fault. The indications are, therefore, that it is a 

 normal fault, with the down-throw to the southward. The amount of 

 stratigraphical throw is estimated at 330 feet (see also p. 15). 



The northern fault is rather obscure. It is occupied by a thick vein of 

 dolomite, and the rock is brecciated in its immediate neighbourhood. The 

 beds on either side dip away from it at 35°, and are strongly dolomitized for 

 about 30' yards, all fossils being obliterated. There is nothing to indicate the 

 direction of hade. The amount of throw must be small, as the fauna on the 

 two sides is almost identical. This being so, the down-throw must be to 

 the north, as otherwise there would be a repetition of beds, which is not 

 the case. 



In the Geological Survey 1-inch map, sheet 102, and memoir to sheets 

 102 and 112, only one fault is recognized, namely, that at Carrickhill tower, 



1 A thick band of light grey limestone occurs under the martello tower. This is 

 repeated twice (as indicated on the map, PI. 11) by the smaller faults, and immediately 

 shows the amount and direction of displacement. 



