SiGERSON — Fish-remains in Alluvial Clay of River Foyle. 217 
The modern map lays down two comparatively large islands, Island- 
more" {recte Oilen-mor," i.e. Great Island) and " Corkan Island," a 
much smaller isle, Island-beg" {recte " Oilen-beg," i.e. Little Island), 
and a yet smaller islet, called '^Yew Island" — that is, four islands, of 
which two are very small. N^ow, the Escheatment Map of Tyrone 
shows five islands in the mile-wide lake. Two of these are small, and 
three comparatively large : and, in addition to these, in Map JS'o. 16, 
a sixth island is placed between Liffer and Strabane. Thus two islands 
have lost their insular character, on account of the water-courses be- 
tween them and the mainland having been filled up. 
Of these two, one was that island lying at the Head of the Lough, 
between Strabane and Lifi'ord. On the Escheatment Map, the Castle of 
Strabane is laid down as close beside the east channel, and its position 
seems to have been selected in order to command the ford. But 
through the choking up of this channel, and the consequent connexion 
of the former island with the Strabane territory, this reason for the se- 
lection of the site is no longer so obvious, seeing that the west channel 
at Lifford, now the only channel of the river, is at a considerable dis- 
tance from Strabane Castle. The interference of man, no doubt, per- 
fected the obstruction of the east channel, as the town of Strabane en- 
larged. One bridge thus suficed, and ground was gained. But it is 
to be remarked that at times of high flood the escapement by one chan- 
nel is scarcely enough, and the river, trying to reconquer its second 
channel, lays part of the town under one or two feet of water, and 
even puts in an appearance beside the Castle. 
Erom this ex-island* at the Head of the Upper Lough, the trans- 
ition to Derry at the other extremity is natural, as there is evidence 
to demonstrate that this place also was once an island. At a first 
glance the statement seems rash, for the valley through which the 
channel should have gone, in order to insulate the city, is now dry and 
firm ground, covered with habitations, and not even subject to those 
inundations which still testify to the former existence of a second 
channel at Strabane. The names, ''Cow-bog" and "Bogside," applied 
to this locality, indicate, however, that there was a well-remembered 
time when this valley was a morass. EoUowing the matter up, in 
order to determine, with as much accuracy as possible, the periods of 
change, so desirable from a geological as well as from a historical 
stand-point, proof is got that two centuries and a half ago the now 
firm ground was a wet, almost impassable bog. For, in 1600, Sir 
Henry Dockwra, in his " JN^arration of Services," wrote : — ''On the 
22nd May wee put the army in order to marche, and, leaving Captaine 
Lancellot Afford at Culmore, with 600 men, to make up the workes, 
wee went to the Derry, 4 miles off upon the river side, a place in the 
* Having since re- visited the locality, I find that the angle of land immediately 
above the meeting of the rivers Finn and Mourn e still retains the name of "the 
island." It is now no longer insulated (exc ept during floods), but men live who 
recollect the existence of the vanished channel. This fact testifies to the accuracy 
of the Escheatment Maps. The angle islet was probably part of a larger isle. 
R. I. A. PROC VOL. I., SEU. II., SCIENCE. 2 F 
