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 tauna 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. XXXIIL, p. 461. 



