Swanston— Silurian Rocks of the Co. Down. 121 
fossils have yet been detected in these beds in Scotland, the evidence depends 
solely upon the peculiar fauna of the Irish beds. Their representatives in the 
Lake Disirict are probably the Knock beds exposed near Ambleside, which 
are considered to be, if not Upper, at least the passage between the Upper 
and the Middle Silurians ( 1 ). If this view of their position be admitted, it is 
evident that the grey grits and conglomerates which come in between this fossil 
band and the black shales at Coalpit Bay naturally fall into the place of the 
Gala Series. The evidence— in the absence of reliable fossils— which they 
afford tends to support this view. The conglomerates referred to constitute a 
well-marked zone, and consist of small pebbles of quarts, flakes of black shale, 
&c., in a grey gritty base. They are well shewn in the quarries at Ballygowan 
and elsewhere throughout the county, and are identical in character to beds 
which mark the base of the Gala Group in Scotland. The remaining rocks of 
this series consist of alternations of coarse grits and slates, void of any 
peculiar feature upon which to fix a horizon. It is this monotonous character 
of strata stretching over such vast areas in both Scotland and Ireland, and 
dipping in the same general direction, that has hitherto baffled all' attempts 
to unravel their sequence, or define their exact position in the Silurian System. 
From the above, however, it is clear that they must be considered as of Gala 
age, the position of which is defined as the top of the Middle Silurian. 
It will perhaps be difficult to define the boundary line between this 
Irish Gala group and the Portaferry beds. The fossils in the latter at Tieve- 
shilly undoubtedly occupy only a black slaty band, the horizon of which in the 
series has not yet been fixed. The few organisms on the surfaces of the grey 
slates near Greyabbey afford very little help, but tend to prove that the rocks 
in which they occur belong to the Gala rather than to the Portaferry series. 
It is to be hoped that the boundary line between these two sets of rocks 
may yet be defined ; but the close resemblance between the unfossiliferous 
beds of both, and the heavy covering of drift by which they are hidden, will 
render this a work of extreme difficulty. 
CONCLUSIONS. 
The Silurian Rocks of County Down are from the foregoing proved to belong 
to several distinct divisions of the system — 
1st. The lowest, exposed at Coalpit Bay, the shales of Ballygrot, Craiga- 
(1). Harkness and Nicholson on the Strata and their Fossil Contents, between the Barrow- 
dale Series of the North of England and the Coniston Flags. — Quartl. Jourl. Geol. Soc., 
London, Vol. XXXIII., p. 461. 
