330 Scientific Proceedings, Royal Dublin Society. 
beneath a stratum of conglomerate. Such strata are conspicuous 
features of the rock-formation of the district. The pit-like 
depressions in the centre of the markings were no doubt occupied 
at one time by projecting pieces of vein-quartz, like those to be 
seen in the beds of conglomerate near at hand. The markings 
almost certainly were formed gradually, about the time that the 
overlying conglomerate was nearly all removed. A stream of 
water containing sand or grit, flowing over the sandstone bed, 
would be arrested and deflected by the few remaining upstanding 
pieces of quartz; and the grinding action of the grit would give 
rise to marks like those to be seen on the rocks. The outline 
of the mark, that is, the form of the arch and the direction of 
its limbs, appears to have been influenced in great measure by 
the size and shape of the obstructing piece of quartz. The drift, 
which at present lies over much of the mountain side and is being 
constantly washed over these rocks, would have furnished excellent 
material for cutting the grooves or markings. In wet weather 
at the present time, there is just such a flow of water and drift 
sand over the rock as would, we believe, be likely to produce the 
markings. Unfortunately there are now no projecting pieces of 
quartz sticking up to deflect the stream; and hence the flow, 
which at present occurs after heavy rain, tends to erase the 
existing marks. 
If we prepare a flat sloping slab of plaster, with projecting 
embedded pebbles, and arrange that a thin stream of water flows 
evenly over it, we can easily demonstrate that particles of sand, 
or heavy, insoluble powder, carried in the stream, as they reach 
and are carried past the obstructing projections, can be made to 
describe figures quite comparable to those cut on the rock at 
Dooghbeg. 
