182 
Proceedings of the Royal Society 
The shock was felt very distinctly at some stations, whilst at 
others only a few miles distant it was not perceived at all. With 
reference to this fact, it seems necessary to remark that at the 
time of the shock there happened to he a keeper on watch in every 
lighthouse, as the earthquake occurred after sunset, at a time when 
the lamps were lighted. 
Two instances will he sufficient to show how capricious the indi- 
cations of the shock were. 
1. At Skerry vore the shock was more severe than at any other 
station, yet not a trace of it was felt at Dhuheartach lighthouse, only 
twenty miles distant. These two lighthouse towers are very similarly 
situated, hoth being absolutely built into solid rocks of small extent 
lying in the open sea, the nearest land to either of them being 
about ten miles distant. 
Being unable to fix the geological ages of the two rocks, I sub- 
mitted specimens of them to Professor Geikie, who kindly examined 
them, and states by letter that he finds “the Dhuheartach rock 
to be a dolerite, identical in external and microscopic characters 
with the more coarsely crystalline dolerites of Mull, Eig, &c. I have 
not the least doubt that it is a portion of the volcanic plateau which 
extends more or less broken from Antrim up into Faroe Islands and 
Iceland. The Skerryvore rock is one of the crystalline schists, and of 
much higher antiquity than the rock of Dhuheartach.” 
2. Again the earthquake was felt at the Sound of Mull lighthouse, 
but was not felt at Ardnamurchan, situated on the other side of the 
Sound of Mull, and only seven miles distant. The Sound of Mull and 
Ardnamurchan lighthouses are both founded on igneous rocks of 
the Miocene age. Judging from this case, one might come to the 
conclusion that the deep cleft or basin of the Sound of Mull had 
cut off the earthquake wave from Ardnamurchan, but on the same 
grounds it should have been cut off at Kyleakin, MacArthur’s Head, 
and other places, and after a careful study of the Chart and 
Geological Map, 1 have come to the conclusion that it is impossible 
to account for the different effects of the earthquake, as observed at 
places within the agitated area, either by the configuration of the land 
or by the directions or positions of the principal lines of fault or of 
the trap dykes, but this much is shown that of twenty -two lighthouse 
observers between Cape Wrath and the Mull of Galloway who were 
